MB-8773: always checkpoint to CAPI unless specified otherwise

XMem enables checkpoint to remote memcached directly. Per discussion
with other engineering members, we decided to keep checkpointing to
CAPI in xmem mode XDCR, unless users specifically want to checkpoint
to remote memcached.

Change-Id: I563c440e6141860b09899935d540a5babacf7c1d
Reviewed-on: http://review.couchbase.org/28100
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>
Tested-by: Junyi Xie <junyi.couchbase@gmail.com>