gtp: remove useless comment on tree / hashtable size

We already use a hashtable, the size is still hardcoded but that
should be easy to fix. I think the hashtable is just fine, no
need to add a rb-tree here.

gtp.c

@@ -80,7 +80,6 @@ struct gtp_instance {
 	struct net_device *dev;
 	struct net_device *real_dev;
 
-	/* FIXME: hash / tree of pdp contexts */
 	unsigned int hash_size;
 	struct hlist_head *tid_hash;
 	struct hlist_head *addr_hash;