Always enable default column feature, remove the configuration

The configuration is introduced to test the default column feature.
Since it has been tested thoroughly for years, remove the
configuration and always enable it.

pinot-core/src/main/java/org/apache/pinot/core/data/manager/config/InstanceDataManagerConfig.java

@@ -41,8 +41,6 @@ public interface InstanceDataManagerConfig {
 
   String getAvgMultiValueCount();
 
-  boolean isEnableDefaultColumns();
-
   boolean isEnableSplitCommit();
 
   boolean isEnableSplitCommitEndWithMetadata();

pinot-core/src/main/java/org/apache/pinot/core/segment/index/loader/IndexLoadingConfig.java

@@ -50,8 +50,6 @@ public class IndexLoadingConfig {
   private Set<String> _bloomFilterColumns = new HashSet<>();
 
   private SegmentVersion _segmentVersion;
-  // This value will remain true only when the empty constructor is invoked.
-  private boolean _enableDefaultColumns = true;
   private ColumnMinMaxValueGeneratorMode _columnMinMaxValueGeneratorMode = ColumnMinMaxValueGeneratorMode.DEFAULT_MODE;
   private int _realtimeAvgMultiValueCount = DEFAULT_REALTIME_AVG_MULTI_VALUE_COUNT;
   private boolean _enableSplitCommit;
@@ -125,8 +123,6 @@ private void extractFromInstanceConfig(@Nonnull InstanceDataManagerConfig instan
       _segmentVersion = SegmentVersion.valueOf(instanceSegmentVersion.toLowerCase());
     }
 
-    _enableDefaultColumns = instanceDataManagerConfig.isEnableDefaultColumns();
-
     _enableSplitCommit = instanceDataManagerConfig.isEnableSplitCommit();
 
     _isRealtimeOffheapAllocation = instanceDataManagerConfig.isRealtimeOffHeapAllocation();
@@ -216,15 +212,13 @@ public void setSegmentVersion(@Nonnull SegmentVersion segmentVersion) {
     _segmentVersion = segmentVersion;
   }
 
-  public boolean isEnableDefaultColumns() {
-    return _enableDefaultColumns;
-  }
-
   public boolean isEnableSplitCommit() {
     return _enableSplitCommit;
   }
 
-  public boolean isEnableSplitCommitEndWithMetadata() { return _enableSplitCommitEndWithMetadata; }
+  public boolean isEnableSplitCommitEndWithMetadata() {
+    return _enableSplitCommitEndWithMetadata;
+  }
 
   public boolean isRealtimeOffheapAllocation() {
     return _isRealtimeOffheapAllocation;

pinot-core/src/main/java/org/apache/pinot/core/segment/index/loader/SegmentPreProcessor.java

@@ -84,7 +84,7 @@ public void process()
 
     try (SegmentDirectory.Writer segmentWriter = _segmentDirectory.createWriter()) {
       // Update default columns according to the schema.
-      if (_indexLoadingConfig.isEnableDefaultColumns() && (_schema != null)) {
+      if (_schema != null) {
         DefaultColumnHandler defaultColumnHandler =
             DefaultColumnHandlerFactory.getDefaultColumnHandler(_indexDir, _schema, _segmentMetadata, segmentWriter);
         defaultColumnHandler.updateDefaultColumns();

pinot-core/src/test/java/org/apache/pinot/core/data/manager/realtime/LLRealtimeSegmentDataManagerTest.java

@@ -654,7 +654,6 @@ private static InstanceDataManagerConfig makeInstanceDataManagerConfig() {
       when(dataManagerConfig.getReadMode()).thenReturn(null);
       when(dataManagerConfig.getAvgMultiValueCount()).thenReturn(null);
       when(dataManagerConfig.getSegmentFormatVersion()).thenReturn(null);
-      when(dataManagerConfig.isEnableDefaultColumns()).thenReturn(false);
       when(dataManagerConfig.isEnableSplitCommit()).thenReturn(false);
       when(dataManagerConfig.isRealtimeOffHeapAllocation()).thenReturn(false);
       return dataManagerConfig;

pinot-server/src/main/java/org/apache/pinot/server/starter/helix/HelixInstanceDataManagerConfig.java

@@ -57,9 +57,6 @@ public class HelixInstanceDataManagerConfig implements InstanceDataManagerConfig
   // Key of the segment format this server can read
   public static final String SEGMENT_FORMAT_VERSION = "segment.format.version";
 
-  // Key of whether to enable default columns
-  private static final String ENABLE_DEFAULT_COLUMNS = "enable.default.columns";
-
   // Key of how many parallel realtime segments can be built.
   // A value of <= 0 indicates unlimited.
   // Unlimited parallel builds can cause high GC pauses during segment builds, causing
@@ -155,11 +152,6 @@ public String getSegmentFormatVersion() {
     return _instanceDataManagerConfiguration.getString(SEGMENT_FORMAT_VERSION);
   }
 
-  @Override
-  public boolean isEnableDefaultColumns() {
-    return _instanceDataManagerConfiguration.getBoolean(ENABLE_DEFAULT_COLUMNS, false);
-  }
-
   @Override
   public boolean isEnableSplitCommit() {
     return _instanceDataManagerConfiguration.getBoolean(ENABLE_SPLIT_COMMIT, false);