* Remove dead code

* MDEV-21675 Data inconsistency after multirow insert rollback

This patch fixes data inconsistencies that happen after rollback of
multirow inserts, with binlog disabled.
For example, statements such as `INSERT INTO t1 VALUES (1,'a'),(1,'b')`
that fail with duplicate key error. In such cases the whole statement
is rolled back. However, with wsrep_emulate_binlog in effect, the
IO_CACHE would not be truncated, and the pending rows events would be
replicated to the rest of the cluster. In the above example, it would
result in row (1,'a') being replicated, whereas locally the statement
is rolled back entirely. Making the cluster inconsistent.
The patch changes the code so that prior to statement rollback,
pending rows event are removed and the stmt cache reset.
That patch also introduces MTR tests that excercise multirow insert
statements for regular, and streaming replication.