txm: carefully handle conflict

When a transaction is in read-confirmed state it must ignore all
prepared changes, and if it actually ignores something - it must
fall to read-view state.

By a mistake the check relied not on actual skipping of a prepared
statement, but on the fact that there is a deleting statement. That
leads to excess conflicts for transactions with read-committed
isolation level.

Fix it by raising a conflict only if a deleting statement is skipped.

Closes #8122
Needed for #7202

NO_DOC=bugfix

(cherry picked from commit 91d6d70f)