MB-18045 correctly support POST /validateCredentials in mixed clusters
MB-18045 correctly support POST /validateCredentials in mixed clusters

...drop this API in 4.5 since UI won't use it anymore and there were
no requests to keep it alive

Change-Id: Iab646495c669d25ff08a92187282135c4ae6015f
Reviewed-on: http://review.couchbase.org/61784
Tested-by: Artem Stemkovski <15a0730a27c69f62ab86f2b6c639fca202166c02@couchbase.com>
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>

ceejatec/ns_server,ceejatec/ns_server,membase/ns_server,membase/ns_server,ceejatec/ns_server,membase/ns_server,membase/ns_server,ceejatec/ns_server,membase/ns_server,membase/ns_server,ceejatec/ns_server,ceejatec/ns_server