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/rooms/edus/mod.rs

@@ -1,11 +1,9 @@
-pub mod presence;
 pub mod read_receipt;
 pub mod typing;
 
-pub trait Data: presence::Data + read_receipt::Data + 'static {}
+pub trait Data: read_receipt::Data + 'static {}
 
 pub struct Service {
-    pub presence: presence::Service,
     pub read_receipt: read_receipt::Service,
     pub typing: typing::Service,
 }