These aren't used in many places, but we've since moved away from
opacity in UI elements like this, so let's just nuke it here for
legibility.
Addresses https://github.com/ppy/osu/discussions/29906.
It didn't ever really make sense for it to be sharing the implementation
details of that (e.g. colouring of primary/dangerous actions), and with
the hotkey display things got outright hacky, so I'm decoupling it
entirely.