Modify WebP default behavior to follow latest decision (generate only WebP for JPEG)

[felixarntz]

As part of https://core.trac.wordpress.org/ticket/55443, it was recently decided to change the default behavior of the WebP implementation back to generating only WebP versions for the sub-sizes of a JPEG upload, rather than both WebP and JPEG versions for every size.

We should align with this in the plugin, i.e. adjust the default value for the webp_uploads_upload_image_mime_transforms filter, so that both image/jpeg and image/webp result in only image/webp.

While the short-term future of the WebP feature is unclear, we should definitely align the plugin implementation with the latest decision made for the core side, as that is the safer default.

[eugene-manuilov]

@felixarntz, I have created PR #527 that removes image/jpeg versions from the transformations list for jpeg uploads. The question is do we need to leave only webp versions for webp uploads as well since generating jpeg-s for webp images doesn't make sense anymore, right?

[eugene-manuilov]

Actually, found one more problem with webp images after implementing changes required for this ticket. Will include it into my PR once I find a way how to fix it.