GEODE-5732: Remove TxCommitMessageTest

After recent move to Mockito, we found that this test doesn't fail when
we break the corresponding product code e.g. changing hasFlagsField() to
always return true should break tests but doesn't.

We verified that the test named like TxCommitMessageBC* do a better job
of checking serialization of TxCommitMessage between product versions.

Co-authored-by: Dan Smith <[email protected]>