Staking: `chill_other` takes stash instead of controller (#2501)

The `chill_other` call is the only staking call that explicitly requires
`controller` in its signature. This PR changes the controller arg to be
the stash instead, with `StakingLedger` then fetching the controller
from storage.

This is not a breaking change per se - the call types do not change, but
is noteworthy as UIs will now want to pass the stash account into
`chill_other` calls, & metadata will reflect this.

Note: This is very low impact. `chill_other` has [hardly ever been
used](https://polkadot.subscan.io/extrinsic?address=&module=staking&call=chill_other&result=all&signedChecked=signed%20only&startDate=&endDate=&startBlock=&timeType=date&version=9431&endBlock=)
on Polkadot - notwithstanding the one called 11 days ago at block
18177457 that was a part of test I did, the last call was made 493 days
ago. Only 2 calls have ever been successful.

Addresses controller deprecation #2500

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Co-authored-by: command-bot <>
Co-authored-by: Gonçalo Pestana <g6pestana@gmail.com>