Remove the unreferenced perspective_parallax demo

[pvcraven]

Summary

Removes arcade/experimental/perspective_parallax.py.

• Not referenced anywhere: no example page, gallery entry, tutorial, test or code refers to it. It was a quick demo of the then-new PerspectiveProjector from Add a perspective projector #2052 (April 2024), and only reformatting has touched it since.
• It doesn't show parallax: the camera orbits a circle of radius 200 every frame. Each frame I rendered shows one building pushed to one side, with blank space where the other layers should be. Its docstring also gave the wrong run command (arcade.examples.perspective_parallax).
• We already have a working parallax example: background_parallax, in the gallery with its own docs page. I ran it and it draws the layered scene correctly.

The cybercity images it used stay, because background_groups uses them.

Checks

• mypy: no issues in 401 files.
• pyright: 0 errors.
• ruff: passes.

The changelog has an entry under Unreleased → Misc Changes.

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