Remove unused code in staking-async

[kianenigma]

• remove the reward-fn from pallet-staking-async. This crate is no longer needed.
• rename ahm-test to integration-tests

[sigurpol]

probably worth backporting to 2512 as well to simplify following staking PRs backporting (e.g. DAP satellite, session keys etc) for 2512 since this will be probably merged first. @kianenigma wdyt?

[kianenigma]

Sure can do!

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/21248501414
Failed job name: test-linux-stable-runtime-benchmarks

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/21248501427
Failed job name: check-runtime-migration

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/21248501414
Failed job name: test-linux-stable

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/21248501427
Failed job name: check-runtime-migration

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
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Failed job name: build-templates-node

[kianenigma]

/cmd prdoc generate --bump major

[kianenigma]

/cmd prdoc --bump major

[sigurpol]

We had some annoyance recently while backporting PRs with major changes that ended up not being strictly major in reality... I guess we might end up backporting this PR as well if we merge it before the pending DAP stuff for phase 1 since it probably makes merging easier and it's no risk. My question here is : do we feel that the removal of reward-fn without public API or anything is really a major change?