Remove unused errinfo function from API

Change-Id: I373bdfbe2d5a6534562039fc698281191533865b
Reviewed-on: http://review.couchbase.org/41640
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>