Remove downloadSiteIcon

[kean]

I'm working towards removing AlamofireImage. The previous downloadSiteIcon variants and uses have been completely replaced with SiteIconView, SiteIconViewModel, and ImageDownloader in all other places. This was the only remaining use case, but I'm not entirely sure why it's using downloadSiteIcon. My understanding is that it's a fallback for loading non-gravatar user avatars if that's still a thing. I think it's going to be safe to just load the image from the URL as is in this case.

To test:

I tested it by commenting out if let gravatar = AvatarURL(url: url) { line and letting the new code run.

Regression Notes

1. Potential unintended areas of impact

2. What I did to test those areas of impact (or what existing automated tests I relied on)

3. What automated tests I added (or what prevented me from doing so)

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

Testing checklist:

• WordPress.com sites and self-hosted Jetpack sites.
• Portrait and landscape orientations.
• Light and dark modes.
• Fonts: Larger, smaller and bold text.
• High contrast.
• VoiceOver.
• Languages with large words or with letters/accents not frequently used in English.
• Right-to-left languages. (Even if translation isn’t complete, formatting should still respect the right-to-left layout)
• iPhone and iPad.
• Multi-tasking: Split view and Slide over. (iPad)

[dangermattic]

	1 Warning
⚠️	This PR is assigned to the milestone 25.5. The due date for this milestone has already passed. Please assign it to a milestone with a later deadline or check whether the release for this milestone has already been finished.

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