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/database/key_value/rooms/edus/mod.rs

@@ -1,4 +1,3 @@
-mod presence;
 mod read_receipt;
 
 use crate::{database::KeyValueDatabase, service};