Deprecates the chill_other extrinsic in staking

[gpestana]

This PR refactors the chill and chill_other extrinsics in staking. The current logic of the chill extrinsic can be deprecated in lieu of chill_other, since the logic of chill is a subset of chill_other. With these changes, all the chilling can be performed by calling the chill extrinsic.

Changes:

1. Removes chill_other extrinsic
2. Refactors chill so that callers can chill own or third-party controller. The caller can pass a optional controller account ID, which has the same logic as the current fn chill_other

pub fn chill(
  origin: OriginFor<T>,
  controller: Option<T::AccountId>,
) -> DispatchResultWithPostInfo

To finish

• Polkadot companion
• Cumulus companion

Closes paritytech/polkadot-sdk#314

[Ank4n]

Seems comment is not correct. I believe this condition is to check if Nominator Preference is undecodeable?

[juangirini]

These are breaking changes for anything using the old chill or chill_other. I suggest to follow the deprecation process proposed here.
I have updated the issue description to track this deprecation process.
In this context we need to keep both extrinsics, but replace any reference to chill in favour of chill_other.
In another PR, once we have confirmed that any examples or tutorials (if any) have been updated, we should add a deprecation warning to chill saying that chill_other should be used instead. This deprecation will be noted in the release log.
Only afterwards we should be good to delete chill, and eventually rename chill_other to chill.

[gpestana]

I'm closing this PR for now as after a few discussions, we think that it's better to avoid having another breaking change in the API at this point. Since this deprecation is mostly cosmetic, let's pause it for now and wait until we have a more stable deprecation process in place.

[Polkadot-Forum]

This pull request has been mentioned on Polkadot Forum. There might be relevant details there:

https://forum.polkadot.network/t/stablising-v15-metadata/2819/9