We should not migrate the reference from `useExisting`. This is because
developers can only use the `useExisting` value as a token. e.g.
```ts
@NgModule({
providers: [
{provide: AppRippleConfig, useValue: rippleOptions},
{provide: MAT_RIPPLE_OPTIONS, useExisting: AppRippleConfig},
]
})
export class AppModule {}
```
In the case above, nothing should be decorated with `@Injectable`. The
`AppRippleConfig` class is just used as a token for injection.
PR Close#33286
4d23b60d09