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()
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