MB-7994: don't cbcollect hash stats

I.e. because it actually walks over all hashtable buckets for all
vbuckets holding locks.

Change-Id: I06c8679fe9c46d05efc266a997c12ab7c922e14f
Reviewed-on: http://review.couchbase.org/25689
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>