fix use std::mem related lint

Signed-off-by: Jason Volk <jason@zemos.net>

src/service/rooms/threads/data.rs

@@ -1,4 +1,4 @@
-use std::mem;
+use std::mem::size_of;
 
 use ruma::{api::client::threads::get_threads::v1::IncludeThreads, OwnedUserId, RoomId, UserId};
 
@@ -35,7 +35,7 @@ impl Data for KeyValueDatabase {
 				.iter_from(&current, true)
 				.take_while(move |(k, _)| k.starts_with(&prefix))
 				.map(move |(pduid, _users)| {
-					let count = utils::u64_from_bytes(&pduid[(mem::size_of::<u64>())..])
+					let count = utils::u64_from_bytes(&pduid[(size_of::<u64>())..])
 						.map_err(|_| Error::bad_database("Invalid pduid in threadid_userids."))?;
 					let mut pdu = services()
 						.rooms