delete 20.0 texture fallback

[RoflikBEST]

Deletes the backup version of the invisible sprite that was added in version 20.0 (756dcc0 - detects invisible objects when loading the sprite texture and replaces them with the default sprite).
This function treated fully transparent sprite areas in user resources as
broken/outdated and automatically replaced them with default sprites.
I think this only gets in the way: many players intentionally use transparent hooks, cursors, and similar user resources.
People have also complained about this behavior from my client.

Checklist

• Tested the change ingame
• Provided screenshots if it is a visual change
• Tested in combination with possibly related configuration options
• Written a unit test (especially base/) or added coverage to integration test
• Considered possible null pointers and out of bounds array indexing
• Changed no physics that affect existing maps
• Tested the change with [ASan+UBSan or valgrind's
  memcheck](https://github.com/ddnet/ddnet/#using-addresssanitizer--undefinedbehavio
  ursanitizer-or-valgrinds-memcheck) (optional)
• I didn't use generative AI to generate more than single-line completions

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[SollyBunny]

This is a revert of 756dcc0 (ddnet#12010). Reverts should be labelled with what commit and preferably PR
A revert is probably a bad way to do this as it will mean future upstream changes will cause conflicts.
The PR says you can make the error/default png invisible but I agree with you that this is a bit silly.

[SollyBunny]

Commit names also don't need (TClient) added onto it

[SollyBunny]

Tested it works, thanks