vinyl: do not double-delete a tx from the conflict manager on rollback

In case of WAL write error, the transaction was rolled back
after successful vy_prepare(). In this case, rollback still tried
to remove the tx from the conflict manager. Double deletion lead
to crash.

Add a test case and do not remove tx twice.

Fixes gh-1681 "vinyl: crash in vy_rollback on ER_WAL_IO"