Removes the Default implementation for RewardDestination (#2402)

This PR removes current default for `RewardDestination`, which may cause
confusion since a ledger should not have a default reward destination:
either it has a reward destination, or something is wrong. It also
changes the `Payee`'s reward destination in storage from `ValueQuery` to
`OptionQuery`.

In addition, it adds a `try_state` check to make sure each bonded ledger
have a valid reward destination.

Closes https://github.com/paritytech/polkadot-sdk/issues/2063

---------

Co-authored-by: command-bot <>
Co-authored-by: Ross Bulat <ross@parity.io>

substrate/frame/staking/src/lib.rs

@@ -407,12 +407,6 @@ pub enum RewardDestination<AccountId> {
 	None,
 }
 
-impl<AccountId> Default for RewardDestination<AccountId> {
-	fn default() -> Self {
-		RewardDestination::Staked
-	}
-}
-
 /// Preference of what happens regarding validation.
 #[derive(PartialEq, Eq, Clone, Encode, Decode, RuntimeDebug, TypeInfo, Default, MaxEncodedLen)]
 pub struct ValidatorPrefs {