[RW-816] Fix: ensure webp derivative images are deleted

[orakili]

Refs: RW-816

This makes sure the webp version of the derivative images are also removed when removing the derivative.

[github-actions]

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Statements:	11.71% ( 1089 / 9296 )
Methods:	11.57% ( 82 / 709 )
Lines:	11.73% ( 1007 / 8587 )

[github-actions]

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Pusher: @orakili, Action: pull_request, Workflow: Run tests