GH-137426: Remove code deprecation of `importlib.abc.ResourceLoader` (GH-137567)

Enough other classes in `importlib.abc` inherit from the class and the deprecation was to redirect people to `TraversableResources`. The documentation now makes it clear the class only exists for backwards compatibility.

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Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>