trx_commit_complete_for_mysql(): Remove some conditions.
We will rely on trx_flush_log_if_needed().

trx_t::must_flush_log_later: Remove. trx_commit_complete_for_mysql()
no longer checks for this.

trx_flush_log_if_needed_low(): Renamed to trx_flush_log_if_needed().
Assert that innodb_flush_log_at_trx_commit!=0 was checked by
the caller and that the transaction is not in XA PREPARE state.

trx_prepare(): Invoke log_write_up_to() directly if needed.