MB-100: killed unused capi_frontend functions

Change-Id: I16873e53ba8cecd8c08a1c500073835bf40db158
Reviewed-on: http://review.couchbase.org/28882
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>