GH-812: Remove redundant union capacity comment

Author: hutiefang76

vector/src/main/java/org/apache/arrow/vector/util/VectorAppender.java

@@ -601,7 +601,6 @@ public ValueVector visit(UnionVector deltaVector, Void value) {
     UnionVector targetUnionVector = (UnionVector) targetVector;
     int newValueCount = targetVector.getValueCount() + deltaVector.getValueCount();
 
-    // Child vectors are created and expanded below; an empty union has zero value capacity.
     while (targetUnionVector.getTypeBuffer().capacity() / UnionVector.TYPE_WIDTH < newValueCount) {
       targetUnionVector.reAlloc();
     }