Remove special handling of _use_global_value atom in config.

Because we've never actually got to use it.

Change-Id: I57a0de264c88a2b6c7acbf5476f0138d189cff47
Reviewed-on: http://review.couchbase.org/33811
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>