Actually call moddata_free_channel(channel);

This was a FIXME item that should have been addressed earlier.
We didn't use any MODDATATYPE_CHANNEL in the core up to now so
this was overlooked. We do use it from now on, though, and it
may very well have been used in 3rd party modules already.

src/api-moddata.c

@@ -125,7 +125,6 @@ void moddata_free_local_client(Client *client)
 	memset(client->moddata, 0, sizeof(client->moddata));
 }
 
-// FIXME: this is never called
 void moddata_free_channel(Channel *channel)
 {
 	ModDataInfo *md;