Update for clippy 1.50 (#2193)

## Issue Addressed

NA

## Proposed Changes

Rust 1.50 has landed 🎉

The shiny new `clippy` peers down upon us mere mortals with disgust. Brutish peasants wrapping our `usize`s in superfluous `Option`s... tsk tsk.

I've performed the goat sacrifice and corrected our evil ways in this PR. Tonight we shall pray that Github Actions bestows the almighty green tick upon us.

## Additional Info

NA


Co-authored-by: realbigsean <seananderson33@gmail.com>
Co-authored-by: Michael Sproul <michael@sigmaprime.io>

beacon_node/eth2_libp2p/src/rpc/rate_limiter.rs

@@ -240,7 +240,7 @@ impl Future for RPCRateLimiter {
     type Output = ();
 
     fn poll(mut self: Pin<&mut Self>, cx: &mut Context) -> Poll<Self::Output> {
-        while let Poll::Ready(_) = self.prune_interval.poll_tick(cx) {
+        while self.prune_interval.poll_tick(cx).is_ready() {
             self.prune();
         }