datalib:course_parent_visible() rework, remove category_parent_visible()

datalib:course_parent_visible() rework, remove category_parent_visible()

Reworked course_parent_visible() to always return in a constant
number of db queries (2 worst case) regardless of nesting depth.

The rewritten version has a small cache, but if you are going to
walk many courses, it's still 1~2 DB queries per category seen,
so the right thing to do is to check it in the caller, as seen
in get_my_courses().