Remove intern calls on FieldMapper#Names

remove internal callas on FieldMappers#Names, we properly reuse FieldMapper, so there is no need to try and call intern in order to reuse the names. This can be heavy with many fields and continuous mapping parsing.
closes #6747

src/main/java/org/elasticsearch/index/mapper/FieldMapper.java

@@ -66,11 +66,11 @@ public Names(String name, String indexName, String indexNameClean, String fullNa
         }
 
         public Names(String name, String indexName, String indexNameClean, String fullName, @Nullable String sourcePath) {
-            this.name = name.intern();
-            this.indexName = indexName.intern();
-            this.indexNameClean = indexNameClean.intern();
-            this.fullName = fullName.intern();
-            this.sourcePath = sourcePath == null ? this.fullName : sourcePath.intern();
+            this.name = name;
+            this.indexName = indexName;
+            this.indexNameClean = indexNameClean;
+            this.fullName = fullName;
+            this.sourcePath = sourcePath == null ? this.fullName : sourcePath;
         }
 
         /**