mod_roster: Change hook type from #roster{} to #roster_item{}

The problem with #roster{} is that every new record entry is also stored
in the mnesia roster table.  Adding the mix_participant_id there makes
no sense because the normal roster items are no MIX channels.  Using
\#roster_item{} for the hook and #roster{} for storing the normal items
seems to be a better idea.

include/mod_roster.hrl

@@ -28,8 +28,7 @@
     ask = none                             :: ask() | '_',
     groups = []                            :: [binary()] | '_',
     askmessage = <<"">>                    :: binary() | '_',
-    xs = []                                :: [fxml:xmlel()] | '_',
-    mix_participant_id = <<>>              :: binary() | '_'
+    xs = []                                :: [fxml:xmlel()] | '_'
 }).
 
 -record(roster_version,