rxrpc: Kill service bundle

Now that the bundle->channel_lock has been eliminated, we don't need the
dummy service bundle anymore.  It's purpose was purely to provide the
channel_lock for service connections.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org