don't detect json-ness of docs for xdcr-xmem

Because it's not affecting anything in xmem path so it's pure waste of
cycles.

Change-Id: Id72dc190da7d456aa83bf7b657d2ea05a7a45d46
Reviewed-on: http://review.couchbase.org/35273
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>