remove half-baked presence implementation

But I'm leaving behind the database state for now in case we want it
back later, so we won't need to do a migration or whatever.

src/service/mod.rs

@@ -64,7 +64,6 @@ impl Services {
                 auth_chain: rooms::auth_chain::Service { db },
                 directory: rooms::directory::Service { db },
                 edus: rooms::edus::Service {
-                    presence: rooms::edus::presence::Service { db },
                     read_receipt: rooms::edus::read_receipt::Service { db },
                     typing: rooms::edus::typing::Service {
                         typing: RwLock::new(BTreeMap::new()),