txm: remove is_prepared_ok argument from memtx_tx_tuple_clarify

This flag actually describes isolation level:
is_prepared_ok == true - READ_COMMITTED,
is_prepared_ok == false - READ_CONFIRMED.

Now it is always calculated as tnx != NULL.
Let memtx_tx_tuple_clarify to calculate it by itself.

No logical changes.

Part of #6930
NO_DOC=refactoring
NO_CHANGELOG=refactoring
NO_TEST=refactoring