Activate clippy::manual_let_else lint (#4889)

## Issue Addressed

#4888

## Proposed Changes

Enabled `clippy::manual_let_else` lint and resolved the warning messages.

validator_client/src/doppelganger_service.rs

@@ -525,9 +525,7 @@ impl DoppelgangerService {
             }
 
             // Resolve the index from the server response back to a public key.
-            let pubkey = if let Some(pubkey) = indices_map.get(&response.index) {
-                pubkey
-            } else {
+            let Some(pubkey) = indices_map.get(&response.index) else {
                 crit!(
                     self.log,
                     "Inconsistent indices map";

validator_client/src/duties_service/sync.rs

@@ -607,9 +607,7 @@ pub async fn fill_in_aggregation_proofs<T: SlotClock + 'static, E: EthSpec>(
 
         // Add to global storage (we add regularly so the proofs can be used ASAP).
         let sync_map = duties_service.sync_duties.committees.read();
-        let committee_duties = if let Some(duties) = sync_map.get(&sync_committee_period) {
-            duties
-        } else {
+        let Some(committee_duties) = sync_map.get(&sync_committee_period) else {
             debug!(
                 log,
                 "Missing sync duties";

validator_client/src/sync_committee_service.rs

@@ -158,13 +158,11 @@ impl<T: SlotClock + 'static, E: EthSpec> SyncCommitteeService<T, E> {
                 .checked_sub(slot_duration / 3)
                 .unwrap_or_else(|| Duration::from_secs(0));
 
-        let slot_duties = if let Some(duties) = self
+        let Some(slot_duties) = self
             .duties_service
             .sync_duties
             .get_duties_for_slot::<E>(slot, &self.duties_service.spec)
-        {
-            duties
-        } else {
+        else {
             debug!(log, "No duties known for slot {}", slot);
             return Ok(());
         };