Comments: Remove VisualEffectView usage

[dvdchr]

Fixes n/a

VisualEffectView was a SwiftUI fallback implementation for materials on iOS 14. Now that we've bumped our minimum version to iOS 15 (via #20917), we can remove this custom implementation.

To test

• Launch the Jetpack app.
• Turn on the Comment moderation update feature flag.
• Go to Reader > any comment thread.
• Verify that the comment header background is displayed in material.

Regression Notes

1. Potential unintended areas of impact
   Should be none. The component is private and only used in one place where the feature is not released.

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

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

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.

UI Changes testing checklist:

• 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)

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr20981-48eea91
	Version	22.7
	Bundle ID	com.jetpack.alpha
	Commit	48eea91
	App Center Build	jetpack-installable-builds #5209

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[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Alpha by scanning the QR code below to install the corresponding build.

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr20981-b8239d4
	Version	22.7
	Bundle ID	org.wordpress.alpha
	Commit	b8239d4
	App Center Build	WPiOS - One-Offs #6175

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[wargcm]

I left a minor comment but it's just a small change so I'll still approve.