Revert "[ISSUE-173][FOLLOWUP] The size of single buffer flush should …

…reach rss.server.flush.cold.storage.threshold.size (#178)" (#179)

This reverts commit ab6be6e.

server/src/main/java/org/apache/uniffle/server/buffer/ShuffleBufferManager.java

@@ -165,9 +165,7 @@ public ShuffleDataResult getShuffleData(
 
   void flushSingleBufferIfNecessary(ShuffleBuffer buffer, String appId,
       int shuffleId, int startPartition, int endPartition) {
-    // When we use multistorage and trigger single buffer flush, the buffer size should be bigger
-    // than rss.server.flush.cold.storage.threshold.size, otherwise cold storage will be useless.
-    if (this.bufferFlushEnabled && buffer.getSize() > this.bufferFlushThreshold) {
+    if (this.bufferFlushEnabled && buffer.getSize() >= this.bufferFlushThreshold) {
       flushBuffer(buffer, appId, shuffleId, startPartition, endPartition);
     }
   }