Do not validate a candidate in candidate selection (#1912)

* Do not validate a candidate in candidate selection

The candidate selection subsystem should not validate a candidate, as
this is done by the backing subsystem on a `Second` request. Otherwise
we validate one candidate twice.

* Update candidate-selection.md

polkadot/roadmap/implementers-guide/src/node/backing/candidate-selection.md

@@ -16,7 +16,6 @@ Input: [`CandidateSelectionMessage`](../../types/overseer-protocol.md#candidate-
 
 Output:
 
-- Validation requests to Validation subsystem
 - [`CandidateBackingMessage`](../../types/overseer-protocol.md#candidate-backing-message)`::Second`
 - Peer set manager: report peers (collators who have misbehaved)