Remove unnecessary getLastFlushLength when checking delete delta files

VectorizedOrcAcidRowBatchReader$ColumnizedDeleteEventRegistry calls OrcAcidUtils.getLastFlushLength for every delete delta file.
Even the comment says:

              // NOTE: Calling last flush length below is more for future-proofing when we have
              // streaming deletes. But currently we don't support streaming deletes, and this can
              // be removed if this becomes a performance issue.

If we have a table with 5 updates (1 base + 5 delta + 5 delete_delta), then for every base + delta dir we will check all of the delete_delta directories, and check the getLastFlushLength method which will result in 6*5=30 unnecessary NN/S3 calls.

We should remove the check as already proposed in the comment.