Closes #790, #792, #803: Delete/Generate next-gen in media not working

[jeawhanlee]

Description

Fixes the delete/generate next-gen link not working.

Fixes #790, #792, #803

Type of change

• [x] Bug fix (non-breaking change which fixes an issue).

Checklists

Feature validation

• [x] I validated all the Acceptance Criteria. If possible, provide sreenshots or videos.
• [x] I triggered all changed lines of code at least once without new errors/warnings/notices.

Code style

• [x] I wrote self-explanatory code about what it does.
• [x] I wrote comments to explain why it does it.
• [x] I named variables and functions explicitely.
• [x] I did not introduce unecessary complexity.

[remyperona]

There is some duplicate here with this PR https://github.com/wp-media/imagify-plugin/pull/804

[jeawhanlee]

@Tabrisrp What do we do about the duplicates?

[Mai-Saad]

@jeawhanlee Thanks for the PR, please find my notes below:

• click generate next-Gen (generating webP /AVIF based on option enabled) :tada: , while delete next gen will delete only webP files even if avif files are there while AVIF option is disabled (in this case, we shall delete both webp and avif). can you please check? steps: 1- on branch 2- enable create avif 3- upload image => avif created 4- switch to trunk and enable create webP 5- click generate webP 6- back to branch => click delete next-gen (AVIF option is disabled) expected: delete both webP and AVIF actual: only webP is deleted

• When should generate/ delete link be displayed is not handled on this PR. will add separate GH about it