Used CountedStorageMap in pallet-staking (#10233)

* Removed counters and used CountedStorageMap instead.

* Little refactoring

* Update frame/staking/src/migrations.rs

Co-authored-by: Shawn Tabrizi <[email protected]>

* Removed redundant code to update counter for validator & nominator.

* Removed redundant code to update counter for validator & nominator.

* Removed unreachable code to inject the hashed prefix for nominator & validator.

* Removed redundant check for nominator & validator count.

* Generated `fn prefix_hash` for `CountedStorageMap`.

* Applied changes from `cargo fmt`

* Possible correct implementation of migration code

* Implemented fn module_prefix, storage_prefix and prefix_hash.

* Removed counted_map.rs

* Renamed `fn storage_prefix` to `storage_counter_prefix`.

* Update frame/support/src/storage/types/counted_map.rs

* Update frame/bags-list/remote-tests/src/snapshot.rs

* Update frame/support/src/storage/types/counted_map.rs

* Fixed errors.

Co-authored-by: Shawn Tabrizi <[email protected]>
Co-authored-by: Guillaume Thiolliere <[email protected]>