Remove out_string (we only support binary)

Change-Id: I65ef0be9adb4265a5df31128f3f1b30e192f9779
Reviewed-on: http://review.couchbase.org/28857
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>