applier: drop a couple of unnecessary arguments

Applier on_rollback and on_wal_write don't need any arguments -
they either work with a global state, or with the signaled applier
stored inside the trigger.

However into on_wal_write() and on_rollback() was passed the
transaction object, unused.

Even if it would be used, it should have been fixed, because soon
these triggers will be fired not only for traditional 'txn'
transactions. They will be used by the synchro request WAL writes
too - they don't have 'transactions'.

Part of #5129