frontend: Remove unused theme parameters and rename MixerScrollArea

[shiina424]

Description

This pull request updates the theme files that were not modified in the following commit:
a42a6c4

The .indicator-mute has been removed from Yami.obt.
The qproperty-meterThickness and qproperty-meterFontScaling have been removed from VolumeMeter.hpp.
The MixerScrollArea has been renamed in Yami.obt.

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Acri, Light, Rachni

• Removed the unused .indicator-mute. (No visual changes.)

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System

• Removed the unused .indicator-mute. (No visual changes.)
• Removed the qproperty-meterThickness and qproperty-meterFontScaling.
  (The following log messages no longer appear.)
  VolumeMeter(0x1dd7f335720) does not have a property named "meterThickness"
VolumeMeter(0x1dd7f335720) does not have a property named "meterFontScaling"

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Classic

• Removed the unused .indicator-mute. (No visual changes.)
• Removed the qproperty-meterThickness.
  (The following log messages no longer appear.)
  VolumeMeter(0x1dd7f335720) does not have a property named "meterThickness"
• Renamed MixerScrollArea.
  • Vertical: Divider lines are now visible in the mixer, matching the appearance up to version 32.0.4.
  • Horizontal: No visual changes.

Before (Version 32.0.4)

Before (Version 32.1.0 rc2)

After (This pull request)

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This is the best I could do with my current knowledge.
If you see a better approach, please feel free to update or close this PR.

Motivation and Context

I noticed these items after the previous pull request and wanted to make the corresponding changes.

How Has This Been Tested?

Tested on Windows 11 25H2 using a local build.

Types of changes

• Tweak (non-breaking change to improve existing functionality)

• Code cleanup (non-breaking change which makes code smaller or more readable)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.