[HtmlSanitizer] Remove MastermindsParser

[javiereguiluz]

Fixes #21284.

No need to mention the addition of the $context argument in parse() method because we already explain the context argument in sanitizeFor() method, which is what users will really use.

[javiereguiluz]

I don't understand the reported issue:

!!  
!!  In ArrayNode.php line 304:
!!                                                                                 
!!    Unrecognized option "exception_controller" under "twig". Available options   
!!    are "auto_reload", "autoescape_service", "autoescape_service_method", "base  
!!    _template_class", "cache", "charset", "date", "debug", "default_path", "fil  
!!    e_name_pattern", "form_themes", "globals", "mailer", "number_format", "opti  
!!    mizations", "paths", "strict_variables".                                     
!!                                                                                  

I started seeing this error in other projects too. Why is it happening now? If Symfony changed this option, it'd do it in a BC-compatible way.