The collator is inactive for weeks and should be removed from the active set.

Forcedly removes a collator candidate from the TopCandidates and
clears all associated storage for the candidate and their
delegators.

Prepares unstaking of the candidates and their delegators stake
which can be unlocked via `unlock_unstaked` after waiting at
least `StakeDuration` many blocks.

Emits `CandidateRemoved`.

# <weight>
- The transaction's complexity is mainly dependent on updating the
  `SelectedCandidates` storage in `select_top_candidates` which in
  return depends on the number of `MaxSelectedCandidates` (N).
- For each N, we read `CandidatePool` from the storage.
---------
Weight: O(N + D) where N is `MaxSelectedCandidates` bounded by
`MaxTopCandidates` and D is the number of delegators of the
collator candidate bounded by `MaxDelegatorsPerCollator`.
- Reads: MaxCollatorCandidateStake, 2 * N * CandidatePool,
  TopCandidates, BlockNumber, D * DelegatorState, D * Unstaking
- Writes: MaxCollatorCandidateStake, N * CandidatePool, D *
  DelegatorState, (D + 1) * Unstaking
- Kills: CandidatePool, DelegatorState for all delegators which only
  delegated to the candidate
# </weight>

- The collator has not been active for a long time.
- People think the collator should be removed from the active group.
- The collator is not doing its job.
- The collator has been inactive for weeks.
- People want to take action and remove the collator.

#27 Remove Inactive Collator