Remove obsolete fallback script now that picture element is supported

[Anuj-Rathore24]

Summary

Remove Fallback Script which was used to replace webp images with jpeg alternatives on older browsers.
Fixes #996

Relevant technical choices

• Remove the fallback File.
• Remove Function webpUploadsDetectFallback and the action associate with this.

[github-actions]

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Co-authored-by: Anuj-Rathore24 <anujrathore24@git.wordpress.org>
Co-authored-by: westonruter <westonruter@git.wordpress.org>
Co-authored-by: adamsilverstein <adamsilverstein@git.wordpress.org>

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[westonruter]

Looks like this test case needs to be removed as well: Test_WebP_Uploads_Load::test_it_should_add_fallback_script_if_content_has_updated_images

[westonruter]

Thank you!