dropped unused capi_utils:get_vbucket_state_doc

Change-Id: I435d4cbcffcb383fc059d65ca421d6843fd9f290
Reviewed-on: http://review.couchbase.org/39510
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>