contrib: get rid of `fxc2`

• I did not remove the -c option for qsb invocation, so it can still use fxc if it is available on the system.
• Currently only Qt uses fxc, and there is no point for other places to use it anymore as we are too late for it and instead dxc should be used instead for SM 6, and for older shader models d3dcompiler can be used at runtime (it can be cached as Qt is doing or not, but it should be fine to not cache since we should not care about performance for old shader models).
• Qt already uses d3dcompiler at runtime to compile HLSL into SM bytecode, and caches the result. Using fxc therefore only increases the very first start up speed.
• This was first added when Qt was using ANGLE. I assume that Qt 5/ANGLE did not have support for using d3dcompiler at runtime. This is no longer relevant anymore, so the reason of keeping fxc no longer holds.

Request review @robUx4 @chub.