[AHM] Revert multi-block election, slashing and staking client pallets (#7939)

Revert the following PRs which we are pulling from release stable2503:
- #7582
- #7424
- #7282

and leave pallet-staking in its pre-AHM state.

We are forking pallet-staking into `pallet-staking` (also referred as
staking-classic, this is the version that will stay on RC) and
`pallet-staking-next` which will live on AH post AHM.

Additional context:
#7858 (comment)

These changes in crate `pallet-staking` will become the staking classic.
The staking next version is worked in the PR #7601.

The `UnappliedSlashes` storage will need to be translated from
`rc::staking-classic` to `ah::staking-next`.
[Bookmarking](https://github.com/paritytech/polkadot-sdk/blob/master/substrate/frame/staking/src/migrations.rs#L91)
the code that can be referred for this.

(cc: @tdimitrov and @seadanda )
1) Revert pallet-staking v17 migration in westend.
- Update `in code storage version` of pallet-staking storage from 17 to
16 (separate PR).
- Update `on chain storage version` of pallet-staking storage from 17 to
16. The storage key for pallet-staking on chain version is
`0x5f3e4907f716ac89b6347d15ececedca4e7b9012096b41c4eb3aaf947f6ea429`
which should be set currently to `0x1100`, and needs to be updated to
`0x1000`.
- After the runtime upgrade with the code from this PR is deployed on
Westend, kill the following storage prefixes under the Pallet prefix
`Staking`:
  - OffenceQueue
  - OffenceQueueEras
  - ProcessingOffence
- UnappliedSlashes: This also exists in staking-classic as a storage map
(one key) and in pre-revert code as double storage map (two keys).
Killing with prefix `UnappliedSlashes` may kill the ones created post
upgrade (but that's okay for westend).
  - VoterSnapshotStatus
  - NextElectionPage
  - ElectableStashes

2) Remove exposure dependency
Worked in the PR: #7936.

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Co-authored-by: Tsvetomir Dimitrov <[email protected]>
Co-authored-by: Maciej <[email protected]>

Author: 3 people