Remove string type hint from webp_uploads_sanitize_image_format() to prevent possible fatal error

[westonruter]

Fixes #1409

Sanitization functions by definition should not have type hints since they are all about doing such type checking at runtime!

[github-actions]

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Co-authored-by: westonruter <westonruter@git.wordpress.org>
Co-authored-by: swissspidy <swissspidy@git.wordpress.org>
Co-authored-by: adamsilverstein <adamsilverstein@git.wordpress.org>
Co-authored-by: adam-ainsworth <adamainsworth@git.wordpress.org>

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[westonruter]

Build for testing: webp-uploads.zip

[adamsilverstein]

Nifty!