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()
can simply check for trx_t::flush_log_later.

trx_t::commit_in_memory(): Treat flush_log_later as input and output.

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_write_serialisation_history(): Move some code from
trx_purge_add_undo_to_history().

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

innobase_commit_ordered_2(): Remove a condition, and do not reset
trx_t::flush_log_later, because it must be passed from
trx_t::commit_in_memory() to trx_commit_complete_for_mysql().