frontend: Remove unused audio mixer widgets

[Warchamp7]

Description

Removes some unused widgets from the old audio mixer.

Motivation and Context

The widgets inside OBSBasic.ui for the Audio Mixer are no longer relevant. The audio mixer has setWidget called on it with the new mixer widget, which is entirely contained within it's class file.

This removes those superfluous widgets from the UI file, as well as deletes the HScrollArea compontent we have, as that is the only place it was actually used.

How Has This Been Tested?

Compiled and everything still worked.

Types of changes

• 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.