memtx: don't set unnecessary conflict

If a transaction writes to a gap that is tracked by some another
transaction, the interval must be (usually) broken into parts,
while new story must be explicitly added to the read set of
reading transaction.

Now both read tracker and conflict trackers are set in this case.
But read tracker is enough in this case - when the writing tx
is prepared it will conflict reading transaction, that's all we
need.

Let's leave only read tracker.

Part of #8122

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring

(cherry picked from commit e6f5090c)