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>

consensus/state_processing/src/per_block_processing/signature_sets.rs

@@ -156,14 +156,14 @@ where
             get_pubkey(proposer_index)
                 .ok_or_else(|| Error::ValidatorUnknown(proposer_index as u64))?,
             spec,
-        )?,
+        ),
         block_header_signature_set(
             state,
             &proposer_slashing.signed_header_2,
             get_pubkey(proposer_index)
                 .ok_or_else(|| Error::ValidatorUnknown(proposer_index as u64))?,
             spec,
-        )?,
+        ),
     ))
 }
 
@@ -173,7 +173,7 @@ fn block_header_signature_set<'a, T: EthSpec>(
     signed_header: &'a SignedBeaconBlockHeader,
     pubkey: Cow<'a, PublicKey>,
     spec: &'a ChainSpec,
-) -> Result<SignatureSet<'a>> {
+) -> SignatureSet<'a> {
     let domain = spec.get_domain(
         signed_header.message.slot.epoch(T::slots_per_epoch()),
         Domain::BeaconProposer,
@@ -183,11 +183,7 @@ fn block_header_signature_set<'a, T: EthSpec>(
 
     let message = signed_header.message.signing_root(domain);
 
-    Ok(SignatureSet::single_pubkey(
-        &signed_header.signature,
-        pubkey,
-        message,
-    ))
+    SignatureSet::single_pubkey(&signed_header.signature, pubkey, message)
 }
 
 /// Returns the signature set for the given `indexed_attestation`.