MB-9559 Remove the invalid check on replica vbucket checkpoint

As we have a separate backfill queue on the replica vbucket, we
don't need the invalid check on the replica vbucket checkpoint.

Change-Id: I890d1e3165a1e58cf93ec5c4a706a4e79fc88067
Reviewed-on: http://review.couchbase.org/30356
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>