Remove migration to v8 since it's already executed (#313)

### Description
<!-- Describe what change this PR is implementing -->

### Types of Changes
<!--- What types of changes does your code introduce? -->
- [ ] Tech Debt (Code improvements)
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Dependency upgrade (A change in substrate or any 3rd party crate
version)

### Migrations and Hooks
<!--- Check the following box with an x if the following applies: -->
- [ ] This change requires a runtime migration.
- [ ] Modifies `on_initialize`
- [ ] Modifies `on_finalize`

### Checklist
<!--- All boxes need to be checked. Follow this checklist before
requiring PR review -->
- [x] Change has been tested locally.
- [ ] Change adds / updates tests.
- [ ] Changelog doc updated.

runtime/cere-dev/src/lib.rs

@@ -1441,13 +1441,6 @@ impl Get<&'static str> for StakingMigrationV11OldPallet {
 	}
 }
 
-pub struct MigrateStakingPalletToV8;
-impl OnRuntimeUpgrade for MigrateStakingPalletToV8 {
-	fn on_runtime_upgrade() -> Weight {
-		pallet_staking::migrations::v8::migrate::<Runtime>()
-	}
-}
-
 /// Unchecked extrinsic type as expected by this runtime.
 pub type UncheckedExtrinsic =
 	generic::UncheckedExtrinsic<Address, RuntimeCall, Signature, SignedExtra>;
@@ -1477,7 +1470,6 @@ parameter_types! {
 
 /// Runtime migrations
 type Migrations = (
-	MigrateStakingPalletToV8,
 	pallet_staking::migrations::v9::InjectValidatorsIntoVoterList<Runtime>,
 	pallet_staking::migrations::v10::MigrateToV10<Runtime>,
 	pallet_staking::migrations::v11::MigrateToV11<Runtime, VoterList, StakingMigrationV11OldPallet>,

runtime/cere/src/lib.rs

@@ -1446,13 +1446,6 @@ impl Get<&'static str> for StakingMigrationV11OldPallet {
 	}
 }
 
-pub struct MigrateStakingPalletToV8;
-impl OnRuntimeUpgrade for MigrateStakingPalletToV8 {
-	fn on_runtime_upgrade() -> Weight {
-		pallet_staking::migrations::v8::migrate::<Runtime>()
-	}
-}
-
 // We don't need to run pallet_balances::pallets::MigrateToTrackInactive or
 // pallet_balances::pallets::MigrateManyToTrackInactive since XCM related only.
 // MigrateToTrackInactive and MigrateManyToTrackInactive simply add CheckingAccount value to
@@ -1483,7 +1476,6 @@ parameter_types! {
 
 /// Runtime migrations
 type Migrations = (
-	MigrateStakingPalletToV8,
 	pallet_staking::migrations::v9::InjectValidatorsIntoVoterList<Runtime>,
 	pallet_staking::migrations::v10::MigrateToV10<Runtime>,
 	pallet_staking::migrations::v11::MigrateToV11<Runtime, VoterList, StakingMigrationV11OldPallet>,