mix_pam: Remove 'Channels' roster group of mix channels

This isn't specified and was probably only meant for debugging. In real
clients having this (forced) 'Channels' group is undesireable in most
cases.

src/mod_mix_pam.erl

@@ -227,7 +227,7 @@ get_mix_roster_items(Acc, {LUser, LServer}) ->
                         name = <<>>,
                         subscription = both,
                         ask = undefined,
-                        groups = [<<"Channels">>],
+                        groups = [],
                         mix_channel = #mix_roster_channel{participant_id = Id}
                     }
                 end, Channels);