Migrate pallet-elections to pallet attribute macro (#9088)

* Migrate elections pallet to pallet attribute macro.

* Metadata fix.

* Update frame/elections/src/lib.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

substrate/frame/elections/src/mock.rs

@@ -20,7 +20,7 @@
 #![cfg(test)]
 
 use frame_support::{
-	StorageValue, StorageMap, parameter_types, assert_ok,
+	parameter_types, assert_ok,
 	traits::{ChangeMembers, Currency, LockIdentifier},
 };
 use sp_core::H256;
@@ -266,7 +266,7 @@ pub(crate) fn new_test_ext_with_candidate_holes() -> sp_io::TestExternalities {
 	let mut t = ExtBuilder::default().build();
 	t.execute_with(|| {
 		<elections::Candidates<Test>>::put(vec![0, 0, 1]);
-		elections::CandidateCount::put(1);
+		elections::CandidateCount::<Test>::put(1);
 		<elections::RegisterInfoOf<Test>>::insert(1, (0, 2));
 	});
 	t