memtx: always check for memtx_tx_manager_use_mvcc_engine flag

Option memtx_tx_manager_use_mvcc_engine changes the behavior of
transaction execution workflow. Usually that is implemented as
direct check of memtx_tx_manager_use_mvcc_engine. But there are
places in the code that rely on the fact that some pointers are
set to not null if the engine is enabled. That's a bit confusing.

Let's always check for memtx_tx_manager_use_mvcc_engine option
when it's needed to determine which workflow must be executed.

Note that checking of memtx_tx_manager_use_mvcc_engine option is
more correct: in case of delete of nothing (delete statement when
a tuple was not found by given key) all the pointers including
old_tuple and new_tuple are null, while logically we still need
to use mvcc execution workflow. Note also that in this case the
mvcc engine does (and must do) almost nothing, so there was no
bug in the previous behaviour.

Part of #8122

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring