Delete high contrast color scheme

This color scheme is a misnomer, and does not actually offer higher contrast than other color schemes. In fact its contrast is generally worse. As a result it hurts more than it helps. A true "high contrast mode" would require changes to the QStyle to make UI elements larger or change their borders to actually offer an improvement for people who need higher contrast than what is offered by Breeze and Breeze Dark.

Accordingly, let's delete this color scheme and migrate current users to Breeze Dark, which is the shipped theme that looks closest to it, and has better contrast in many ways.

BUG: 352506 BUG: 442286 FIXED-IN: 5.24

cc @teams/usability

Edited by Nate Graham

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