xdcr: removed some useless code read_validate_checkpoint path

Change-Id: I02d26a240e11965869a5f799182dd3ffaa1ba331
Reviewed-on: http://review.couchbase.org/38707
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>