MB-9096: killed cluster_compat_mode:is_cluster_20

And all code paths that implemented 1.8.x backwards compat based on
on it's result

Change-Id: I22b5ce48d219defbef23b09ee0866b879d66edfe
Reviewed-on: http://review.couchbase.org/29314
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>