Remove the RoleProvider.getRoles() method

Remove the obsolete getRoles() method in the RoleProvider and RoleDirectoryService interfaces.

This method has not been used since the introduction of findRoles(...) which is used by both internal role providers and roleproviders for LMS integration (Sakai, Moodle, Brightspace) and the CustomRoleProvider (MH-12181).

https://github.com/opencast/opencast/pull/1046#issuecomment-533716904