Remove SlotsCrowdloanIndexMigration & InjectValidatorsIntoVoterList (#5374) (#5379)

* Remove SlotsCrowdloanIndexMigration

* Remove slots index migration code

* Remove crowdloan index migration code

* remove reference to migration

* rustfmt

* westend: remove unused import OnRuntimeUpgrade

* kusama: remove unused import OnRuntimeUpgrade

* polkadot: remove unused import OnRuntimeUpgrade

* Revert "Remove crowdloan index migration code"

This reverts commit fe84b59e3133e950740d911fb019e6002f60d5a3.

* Revert "Remove slots index migration code"

This reverts commit 62c5a28e21d2949189bdb52835cc3a44f6e9c561.

* Revert "remove reference to migration"

This reverts commit 1f8806c010210b638cc59979ab873619910596c0.

* Remove InjectValidatorsIntoVoterList migration