enable unreachable_pub lint

This causes some other lints to start firing too (which is good), but
I'm going to fix them in follow-up commits to keep things organized.

src/service/account_data/data.rs

@@ -7,7 +7,7 @@ use ruma::{
     RoomId, UserId,
 };
 
-pub trait Data: Send + Sync {
+pub(crate) trait Data: Send + Sync {
     /// Places one event in the account data of the user and removes the previous entry.
     fn update(
         &self,