Clean Finality Verifier Pallet (#804)

* Remove unused Config types from `pallet-finality-verifier`

* Remove unused AncestryChecker trait

* Remove ancestry proof parameter from relayer calls

* Update docs to reflect current state of pallet

* Remove mock ancestry checker

* Remove unused error

* Write headers outside of function used for authority set changes

* Move justification verification into helper function

* Add documentation suggestions

Co-authored-by: Tomasz Drwięga <[email protected]>

* Clean up module level documentation a bit

Co-authored-by: Tomasz Drwięga <[email protected]>