Clears Old Storage for Session pallet (#7132)

* Fixes migration for Session pallet

* Moves migration to polkadot

* Minor change

* Fixes test

* ".git/.scripts/commands/fmt/fmt.sh"

* Allow dead_code for test

* removes test

* Minor change

* Fixes build

* Import vec for try-runtime

* Addresses review comment

* ".git/.scripts/commands/fmt/fmt.sh"

* Addresses review comment

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Co-authored-by: command-bot <>