Delete obsolete -platform xcb args

[uklotzde]

Enforcing Mixxx to use XWayland on Wayland now causes bugs like #11576.

I also don't experience any issue with the legacy GL/GLSL waveform shaders.

[uklotzde]

Otherwise I would need to manually patch this out.

[Swiftb0y]

Thank you. I forgot we decided to do this.

[Swiftb0y]

just fyi there is a small visual glitch with the (GL) waveform variants. My guess is that some blend setting or similar is wrong resulting in out-of-range values because the resulting waveforms look blown out onscreen but the screenshot produced by gnome looks entirely different. The difference probably comes from the GPU just clamping the values while the compositor (or whatever is responsible for screenshots) just ignores the integer part.

In either case, nothing blocking this PR but we'll probably get reports of people being on the GL waveforms from 2.3 even though they should switch to the (all-shaders) anyways. I guess would be a valid reason to remove the GL waveforms altogether?

[daschuer]

LGTM, Thank you.