Remove FeatureRoster from ConnectionFeatures

Now that GlobalContactManager will upgrade the connection object
when we request a full contact roster (in the "invite contact" dialog)
we don't need to fetch the whole roster on load.
(see http://commits.kde.org/telepathy-common-internals/9cc7ea57c830d345bab5377208058fb47b428b69)

This should result in faster load times.