Easy PR: Fix warnings from latest nightly (#14195)

* unneeded mut

* remove needless borrows

substrate/primitives/npos-elections/src/lib.rs

@@ -470,7 +470,7 @@ pub fn to_support_map<AccountId: IdentifierT>(
 	// build support struct.
 	for StakedAssignment { who, distribution } in assignments.iter() {
 		for (c, weight_extended) in distribution.iter() {
-			let mut support = supports.entry(c.clone()).or_default();
+			let support = supports.entry(c.clone()).or_default();
 			support.total = support.total.saturating_add(*weight_extended);
 			support.voters.push((who.clone(), *weight_extended));
 		}