Enable clippy::useless_conversion (#8724)

This PR enables the
[`clippy::useless_conversion`](https://rust-lang.github.io/rust-clippy/master/index.html#/useless_conversion)
rule and fixes the outstanding violations.

Release Notes:

- N/A

crates/channel/src/channel_buffer.rs

@@ -126,7 +126,7 @@ impl ChannelBuffer {
         for (_, old_collaborator) in &self.collaborators {
             if !new_collaborators.contains_key(&old_collaborator.peer_id) {
                 self.buffer.update(cx, |buffer, cx| {
-                    buffer.remove_peer(old_collaborator.replica_id as u16, cx)
+                    buffer.remove_peer(old_collaborator.replica_id, cx)
                 });
             }
         }

crates/channel/src/channel_chat.rs

@@ -681,7 +681,7 @@ pub fn mentions_to_proto(mentions: &[(Range<usize>, UserId)]) -> Vec<proto::Chat
                 start: range.start as u64,
                 end: range.end as u64,
             }),
-            user_id: *user_id as u64,
+            user_id: *user_id,
         })
         .collect()
 }