Revert forced colors color-scheme on audio and video players

[Eugene (OEvgeny)]

Reverts #5906

Changelog Entry

None: #5906's entry was unreleased and is removed.

Description

In real dark high contrast themes, such as "Aquatic" on Windows 11, the color-scheme: light dark workaround from #5906 hides the play, mute, and "more options" icons of audio players and voice messages on a fresh render, while players without it show them. This PR reverts the workaround, its 2 tests, and its changelog entry.

Design

• Revert instead of replacing the workaround: on a fresh render, Chromium's default media controls already match the forced colors palette; the remaining issue (icons not updated after switching palettes) is a Chromium bug.
• The 2 tests are removed, not rewritten: forced colors emulation (DevTools/CDP) does not reach the media control icons, so html2 tests cannot show the real rendering.

Specific Changes

• AudioContent.ts, VideoContent.ts: removed the forced colors color-scheme rule
• Removed attachment/audio.forcedColors.dark.html and attachment/video.forcedColors.dark.html with their snapshots
• CHANGELOG.md: removed Show audio player icons in dark high contrast themes #5906's entry

• I have added tests and executed them locally
• I have updated CHANGELOG.md
• I have updated documentation

Review Checklist

This section is for contributors to review your work.

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)

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