cleanup(batcher): removing the non-applicable TODOs comments (#834)

* fix(batcher): fix logged messge when exception occurs post processing

This commit addresses logged message while batch post
processing.

Currently, if a batch is successful and some exception
occurs during processing then the Batcher generates an exception message
with "Batching finished with 3 batches failed" which is not an accurate
message.

After this commit, for the same scenario, the message will be
logged with "Batching finished with 3 partial failures...".

* removed unnecessary batcher message changes, Also reverted the BatcherStats method name to `recordBatchElementsCompletion()`.

gax/src/main/java/com/google/api/gax/batching/BatcherImpl.java

@@ -256,8 +256,6 @@ void add(ElementT element, SettableApiFuture<ElementResultT> result) {
       byteCounter += descriptor.countBytes(element);
     }
 
-    // TODO: Ensure that all results are resolved in case the descriptor that causes it to
-    //  process all results or throw an exception during processing
     void onBatchSuccess(ResponseT response) {
       try {
         descriptor.splitResponse(response, results);

gax/src/test/java/com/google/api/gax/batching/BatcherImplTest.java

@@ -509,7 +509,6 @@ public void splitResponse(
                 + "3 entries that failed with: 3 ArithmeticException.");
   }
 
-  // TODO(rahulkql): fix this test with follow up PR related to exception in splitResponse.
   @Test
   public void testPartialFailureInResultProcessing() throws Exception {
     final Queue<RuntimeException> queue = Queues.newArrayBlockingQueue(3);