CBD-926: Remove SASL_CB_GETCONF (not used)

Change-Id: I88669499f7c436955b029496da0c240df54dbcb3
Reviewed-on: http://review.couchbase.org/25356
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>