Remove ValidateFromChainState (#5707)

# Description

This PR removes the
`CandidateValidationMessage::ValidateFromChainState`, which was
previously used by backing, but is no longer relevant since initial
async backing implementation
https://github.com/paritytech/polkadot/pull/5557.

Fixes https://github.com/paritytech/polkadot-sdk/issues/5643

## Integration

This change should not affect downstream projects since
`ValidateFromChainState` was already unused.

## Review Notes

- Removed all occurrences of `ValidateFromChainState`.
- Moved utility functions, previously used in candidate validation tests
and malus, exclusively to candidate validation tests as they are no
longer used in malus.
- Deleted the
`polkadot_parachain_candidate_validation_validate_from_chain_state`
metric from Prometheus.
- Removed `Spawner` from `ReplaceValidationResult` in malus’
interceptors.
- `fake_validation_error` was only used for `ValidateFromChainState`
handling, while other cases directly used
`InvalidCandidate::InvalidOutputs`. It has been replaced with
`fake_validation_error`, with a fallback to
`InvalidCandidate::InvalidOutputs`.
- Updated overseer’s minimal example to replace `ValidateFromChainState`
with `ValidateFromExhaustive`.