replication: update stale comments regarding synchro filtering

Starting with commit deca9749
("replication: unify replication filtering with and without elections")
The filter works always, even when elections are turned off.

Reflect that in the comments for applier_synchro_filter_tx and
txn_limbo_is_replica_outdated.

Follow-up #6133

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring