MDL-54035 accesslib: separate role definition cache clear

Thanks to MDL-49398, we can separate the combined user session and role
definition cache clearing function into two separate functions. At the
same time, we want to identify and remove mark_dirty() calls that were
added for role definition changes but were incorrectly left behind.

Change highlights:
 - Remove unnecessary mark_dirty() calls performed after
   assign_capability(), unassign_capability(), delete_role(),
   deleted contexts, brand new contexts
 - Move role definition cache clear from the user-centric
   accesslib_clear_all_caches() to the newly created,
   role-dedicated accesslib_reset_role_cache()

message/tests/api_test.php

@@ -254,7 +254,6 @@ class core_message_api_testcase extends core_message_messagelib_testcase {
         // Remove the viewparticipants capability from one of the courses.
         $course5context = context_course::instance($course5->id);
         assign_capability('moodle/course:viewparticipants', CAP_PROHIBIT, $role->id, $course5context->id);
-        $course5context->mark_dirty();
 
         // Perform a search.
         list($contacts, $courses, $noncontacts) = \core_message\api::search_users($user1->id, 'search');