frontend: Remove Exit button

[Warchamp7]

Description

Remove the Exit button from the Controls dock

Motivation and Context

I don't know a single other application that has an exit button in the main UI.

The Controls dock is something I'd like to fully replace one day but in the meantime this helps remove some clutter from it.

How Has This Been Tested?

Compiled

Types of changes

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

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.