gossmap: delete channels from map when deleted from store

This addresses a crash where a deleted channel is added to the
txout_failures map, and temporarily ignored, but a reference is still
kept in the gossmap.  When the channel expires fromt the txout_failures
map, it is once again looked up in the gossmap, leading to a crash while
accessing a deleted entry.

Fixes ElementsProject#7689

Changelog-Fixed: Fixes a crash in gossipd accessing a deleted channel.

gossipd/gossmap_manage.c

@@ -318,6 +318,7 @@ static void remove_channel(struct gossmap_manage *gm,
 					      GOSSIP_STORE_DYING_BIT,
 					      WIRE_NODE_ANNOUNCEMENT);
 	}
+	gossmap_remove_chan(gossmap, chan);
 }
 
 static u32 get_timestamp(struct gossmap *gossmap,