crypto: remove useless call to self.cache in Store::users_for_key_query

The store cache can be filled lazily when it's actually needed. It's not needed in this
function here, and may be needed in another function the caller of this function may call
later. No need to preemptively fill the cache here.

crates/matrix-sdk-crypto/src/store/mod.rs

@@ -1168,9 +1168,6 @@ impl Store {
     pub(crate) async fn users_for_key_query(
         &self,
     ) -> Result<(HashSet<OwnedUserId>, SequenceNumber)> {
-        // Make sure the tracked users set is up to date.
-        let _cache = self.cache().await?;
-
         Ok(self.inner.users_for_key_query.lock().await.users_for_key_query())
     }