After recent improvements to Lint-related classes, there are now a few cleanup opportunities relating to MandatoryWarningHandler.

1. The new LintWarning class provides access to the associated lint category. This means that the LintCategory being passed to the MandatoryWarningHandler constructor is no longer being used.

2. The prefix string being passed to the MandatoryWarningHandler constructor can (in most cases) instead be inferred as well.

3. The field Check.sunApiHandler (which has type MandatoryWarningHandler) is no longer used and can be removed.