replication: tolerate synchro rollback during final join

Both box_process_register and box_process_join had guards ensuring that
not a single rollback occured for transactions residing in WAL around
replica's _cluster registration.
Both functions would error on a rollback and make the replica retry
final join.

The reason for that was that replica couldn't process synchronous
transactions correctly during final join, because it applied the final
join stream row-by-row.

This path with retrying final join was a dead end, because even if
master manages to receive no ROLLBACK messages around N-th retry of
box.space._cluster:insert{}, replica would still have to receive and
process all the data dating back to its first _cluster registration
attempt.
In other words, the guard against sending synchronous rows to the
replica didn't work.

Let's remove the guard altogether, since now replica is capable of
processing synchronous txs in final join stream and even retrying final
join in case the _cluster registration was rolled back.

Closes #5566