Currently there's only one place where applier_synchro_filter_tx
accesses limbo state under a latch: this place is
txn_limbo_is_replica_outdated. Soon there will be more accesses to limbo
parameters and all of them should be guarded as well.

Let's simplify things a bit and guard the whole synchro_filter_tx with
the limbo latch.

While we are at it remove txn_limbo_is_replica_outdated as not needed
anymore.

Part-of #7286

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring