Remove domain group create function from core

I hit a problem with an infinite loop when the multisite module is in use.

The loop is coming from CRM_Core_Domain:getGroupID - if multisite is enabled and there is no domain group it tries to create one but this goes into a loop.

My fix is to remove this code from core & let the multisite / users of multisite be responsible for the presence of a domain group on a multisite. NB, multisite has become less keen for all sites to have a domain group as it causes performance pain on the top level site for no ACL impact. Hence, we now advocate no domain group for the L1 site.