_gabble_roster_item_to_message: remove some unused functionality
_gabble_roster_item_to_message: remove some unused functionality

- the ability to return the LmMessageNode (we don't use it any more)
- the ability to build a message representing the unmodified state of a
  contact on the roster (likewise)

Ziemin/telepathy-gabble,mlundblad/telepathy-gabble,jku/telepathy-gabble,jku/telepathy-gabble,mlundblad/telepathy-gabble,mlundblad/telepathy-gabble,Ziemin/telepathy-gabble,Ziemin/telepathy-gabble,jku/telepathy-gabble,Ziemin/telepathy-gabble