core(image-elements): drop spritesheet logic

[patrickhulce]

Summary
We previously had some broken logic to address spritesheets in CSS for our responsive images but it has several flaws. I think we should just drop it and ignore CSS background images in our responsive image audit.

Here's why:

1. It didn't work (Properly Size Images 54 KB to 53 KB #8927). This one is obviously fixable but I don't think we should for the other reasons below :)
2. Doing responsive images in CSS is far more difficult and typically less impactful than responsive images in HTML. i.e. the opportunity to effort ratio is much lower here than other areas we should be wanting to spend time fixing things.
3. A false negative is better than a false positive.
4. It fails to account for additional usages of the sprites that might be hidden on interaction (thought to be original cause of Properly Size Images 54 KB to 53 KB #8927).
5. It interferes with the desired audit-agnostic property of gatherers, and we would need to expose quite a bit of additional CSS computed style data on the artifact to allow the audit to determine this properly.
6. It fails to account for various background size and repeat scenarios where the sizing is intentional and most beneficial for the user.

Related Issues/PRs
fixes #8927

[brendankenny]

@paulirish wanted a go at this

[patrickhulce]

To clarify, @paulirish wanted to review this, or he wanted to make a case for keeping the spritesheet logic? :)

[brendankenny]

My understanding is that he thought there was more insight the debugger protocol could provide, but maybe I'm misremembering. You were in that chat too :)

[patrickhulce]

Oh I thought that was about #8493. This PR doesn't deal with stylesheet content really, just image elements that have background set to image which is problematic even when we have the complete stylesheet content. But maybe I'm mixing up my conversations 😆

[paulirish]

Oh I thought that was about #8493.

Yup. I was talking about that one. :)

this LG.

[brendankenny]

so is the main thing that's happening is that ImageElements now includes all background-image images (some were filtered out before for appearing to be sprite sheets or whatever) while uses-responsive-images is now going to ignore all images in CSS?

[patrickhulce]

ImageElements now includes all background-image images (some were filtered out before for appearing to be sprite sheets or whatever) while uses-responsive-images is now going to ignore all images in CSS?

Bingo

[brendankenny]

LGTM 👍