chore(oidc): remove legacy storage methods (#10061)

# Which Problems Are Solved

Stabilize the optimized introspection code and cleanup unused code.

# How the Problems Are Solved

- `oidc_legacy_introspection` feature flag is removed and reserved.
- `OPStorage` which are no longer needed have their bodies removed.
- The method definitions need to remain in place so the interface
remains implemented.
  - A panic is thrown in case any such method is still called

# Additional Changes

- A number of `OPStorage` methods related to token creation were already
unused. These are also cleaned up.

# Additional Context

- Closes #10027 
- #7822

---------

Co-authored-by: Livio Spring <livio.a@gmail.com>

internal/api/oidc/device_auth.go

@@ -88,6 +88,6 @@ func (o *OPStorage) StoreDeviceAuthorization(ctx context.Context, clientID, devi
 	return err
 }
 
-func (o *OPStorage) GetDeviceAuthorizatonState(ctx context.Context, _, deviceCode string) (state *op.DeviceAuthorizationState, err error) {
-	return nil, nil
+func (o *OPStorage) GetDeviceAuthorizatonState(context.Context, string, string) (*op.DeviceAuthorizationState, error) {
+	panic(o.panicErr("GetDeviceAuthorizatonState"))
 }