Fix clippy lints for rust 1.62 (#3300)

## Issue Addressed

Fixes some new clippy lints after the last rust release
### Lints fixed for the curious:
- [cast_abs_to_unsigned](https://rust-lang.github.io/rust-clippy/master/index.html#cast_abs_to_unsigned)
- [map_identity](https://rust-lang.github.io/rust-clippy/master/index.html#map_identity) 
- [let_unit_value](https://rust-lang.github.io/rust-clippy/master/index.html#let_unit_value)
- [crate_in_macro_def](https://rust-lang.github.io/rust-clippy/master/index.html#crate_in_macro_def) 
- [extra_unused_lifetimes](https://rust-lang.github.io/rust-clippy/master/index.html#extra_unused_lifetimes)
- [format_push_string](https://rust-lang.github.io/rust-clippy/master/index.html#format_push_string)

beacon_node/network/src/beacon_processor/worker/gossip_methods.rs

@@ -45,7 +45,7 @@ struct VerifiedUnaggregate<T: BeaconChainTypes> {
 
 /// This implementation allows `Self` to be imported to fork choice and other functions on the
 /// `BeaconChain`.
-impl<'a, T: BeaconChainTypes> VerifiedAttestation<T> for VerifiedUnaggregate<T> {
+impl<T: BeaconChainTypes> VerifiedAttestation<T> for VerifiedUnaggregate<T> {
     fn attestation(&self) -> &Attestation<T::EthSpec> {
         &self.attestation
     }
@@ -72,7 +72,7 @@ struct VerifiedAggregate<T: BeaconChainTypes> {
 
 /// This implementation allows `Self` to be imported to fork choice and other functions on the
 /// `BeaconChain`.
-impl<'a, T: BeaconChainTypes> VerifiedAttestation<T> for VerifiedAggregate<T> {
+impl<T: BeaconChainTypes> VerifiedAttestation<T> for VerifiedAggregate<T> {
     fn attestation(&self) -> &Attestation<T::EthSpec> {
         &self.signed_aggregate.message.aggregate
     }

beacon_node/network/src/subnet_service/attestation_subnets.rs

@@ -623,7 +623,7 @@ impl<T: BeaconChainTypes> Stream for AttestationService<T> {
         // process any known validator expiries
         match self.known_validators.poll_next_unpin(cx) {
             Poll::Ready(Some(Ok(_validator_index))) => {
-                let _ = self.handle_known_validator_expiry();
+                self.handle_known_validator_expiry();
             }
             Poll::Ready(Some(Err(e))) => {
                 error!(self.log, "Failed to check for random subnet cycles"; "error"=> e);