fix(staking): add remove old UnbondingID func (#29)

Author: gezerui0124

x/staking/keeper/unbonding.go

@@ -25,6 +25,12 @@ func (k Keeper) GetOldUnbondingID(ctx context.Context) (unbondingID uint64, err
 	return unbondingID, err
 }
 
+func (k Keeper) RemoveOldUnbondingID(ctx context.Context) error {
+	store := k.storeService.OpenKVStore(ctx)
+	// Old UnbondingID used same key as period delegation, which is not used independently.
+	return store.Delete(types.PeriodDelegationKey)
+}
+
 func (k Keeper) GetUnbondingID(ctx context.Context) (unbondingID uint64, err error) {
 	store := k.storeService.OpenKVStore(ctx)
 	bz, err := store.Get(types.UnbondingIDKey)