[libcontacts] Avoid fallback to non-name details when grouping

Including this fallback option leads to the following undesirable scenario:
  1. contact details yield a group which is not in the allGroups set
  2. the contact is originally grouped in the '#' group
  3. when the contact's addresses are later retrieved, the fallback code re-classifies the contact under a group derived from a non-name property of the contact
  4. the contact is no longer visible under the original group; the new group has no connection to the contact's displayed name, and cannot be reliably predicted by the user