refactor: remove deprecated pallet-dapp-staking-migration

[ipapandinas]

Pull Request Summary

• remove pallet-dapp-staking-migration code
• remove deprecated new_migration method for SingularStakingInfo
• fix unused method warning for try-runtime feature

[github-actions]

Package	Line Rate	Branch Rate	Health
chain-extensions/types/assets/src	0%	0%	❌
pallets/unified-accounts/src	86%	0%	✔
pallets/astar-xcm-benchmarks/src/generic	100%	0%	✔
precompiles/assets-erc20/src	78%	0%	➖
precompiles/xcm/src	71%	0%	➖
pallets/collective-proxy/src	86%	0%	✔
pallets/static-price-provider/src	85%	0%	✔
chain-extensions/pallet-assets/src	56%	0%	➖
chain-extensions/unified-accounts/src	0%	0%	❌
precompiles/dapp-staking-v3/src	90%	0%	✔
pallets/dapp-staking-v3/src/benchmarking	98%	0%	✔
pallets/dynamic-evm-base-fee/src	89%	0%	✔
pallets/dapp-staking-v3/rpc/runtime-api/src	0%	0%	❌
pallets/xc-asset-config/src	50%	0%	➖
precompiles/substrate-ecdsa/src	74%	0%	➖
chain-extensions/types/unified-accounts/src	0%	0%	❌
precompiles/dispatch-lockdrop/src	86%	0%	✔
pallets/dapp-staking-v3/src	84%	0%	✔
pallets/dapp-staking-v3/src/test	0%	0%	❌
pallets/price-aggregator/src	82%	0%	✔
precompiles/dapp-staking-v3/src/test	0%	0%	❌
precompiles/unified-accounts/src	100%	0%	✔
primitives/src	59%	0%	➖
pallets/astar-xcm-benchmarks/src/fungible	100%	0%	✔
pallets/collator-selection/src	92%	0%	✔
pallets/ethereum-checked/src	74%	0%	➖
primitives/src/xcm	65%	0%	➖
pallets/inflation/src	93%	0%	✔
precompiles/sr25519/src	64%	0%	➖
pallets/astar-xcm-benchmarks/src	86%	0%	✔
Summary	80% (3565 / 4478)	0% (0 / 0)	➖

Minimum allowed line rate is 50%