Remove unfinished array and bindless texture atlas stubs

[pvcraven]

Summary

Removes arcade/texture_atlas/atlas_array.py and arcade/texture_atlas/atlas_bindless.py.

Why

• They were empty placeholders. Einar added them in Reorganize texture_atlas module #1951 (January 2024) as stubs, marked "THIS IS WORK IN PROGRESS. DO NOT USE." Every later commit to them was a repo-wide cleanup (formatting, import sorting, type syntax).
• Nothing used them. Nothing in arcade, the tests, or the docs imports them, and arcade.texture_atlas doesn't export them.
• They couldn't work. Neither class implements any of TextureAtlasBase's 19 abstract methods, so creating one raises TypeError.
• They didn't import. On Python 3.10–3.13, both raised NameError: name 'ArcadeContext' is not defined on import. "Drop 3.9" (Drop 3.9 #2622) removed their from __future__ import annotations, and their __init__ signatures use ArcadeContext, which is only imported under TYPE_CHECKING. Python 3.14's lazy annotations would hide this, and nothing imports these modules, so nobody noticed.

They can be restored from git history if the array or bindless atlas work resumes.

Checks

• I searched the repo for atlas_array, atlas_bindless, TextureArrayAtlas and BindlessTextureAtlas: there are no references left.
• I also checked every other module that uses TYPE_CHECKING without from __future__ import annotations (including the WebGL backend, which can't be imported outside a browser) for the same kind of break, by parsing them: none have it.
• arcade.texture_atlas still imports and exports the same names, and util/update_quick_index.py runs cleanly.
• The collected tests are identical before and after (1362). Results: 1359 passed; the 3 failures are the render tests that only fail on my machine.
• Added a changelog entry under Misc Changes.

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