Remove unused extrinsic from dapp staking

[Dinonard]

Pull Request Summary

Remove unused extrinsic from dApp staking - before it was used to fix an issue but it's no longer relevant.
Also removed a resolved TODO.

[github-actions]

Package	Line Rate	Branch Rate	Health
pallets/dapp-staking/rpc/runtime-api/src	0%	0%	❌
chain-extensions/pallet-assets/src	56%	0%	➖
pallets/inflation/src	93%	0%	✔
pallets/astar-xcm-benchmarks/src	86%	0%	✔
precompiles/dapp-staking/src/test	0%	0%	❌
primitives/src	57%	0%	➖
pallets/collator-selection/src	92%	0%	✔
pallets/collective-proxy/src	86%	0%	✔
pallets/static-price-provider/src	85%	0%	✔
pallets/xc-asset-config/src	50%	0%	➖
pallets/price-aggregator/src	82%	0%	✔
pallets/astar-xcm-benchmarks/src/generic	100%	0%	✔
precompiles/assets-erc20/src	78%	0%	➖
pallets/dapp-staking/src/test	0%	0%	❌
precompiles/unified-accounts/src	100%	0%	✔
primitives/src/xcm	65%	0%	➖
pallets/dynamic-evm-base-fee/src	89%	0%	✔
pallets/astar-xcm-benchmarks/src/fungible	100%	0%	✔
pallets/unified-accounts/src	86%	0%	✔
pallets/dapp-staking/src/benchmarking	98%	0%	✔
chain-extensions/unified-accounts/src	0%	0%	❌
pallets/ethereum-checked/src	74%	0%	➖
precompiles/dapp-staking/src	90%	0%	✔
precompiles/sr25519/src	64%	0%	➖
chain-extensions/types/unified-accounts/src	0%	0%	❌
precompiles/xcm/src	71%	0%	➖
pallets/dapp-staking/src	83%	0%	✔
precompiles/substrate-ecdsa/src	74%	0%	➖
precompiles/dispatch-lockdrop/src	86%	0%	✔
chain-extensions/types/assets/src	0%	0%	❌
Summary	79% (3634 / 4606)	0% (0 / 0)	➖

Minimum allowed line rate is 50%