When the D3D11 backend was first written, SPIRV-Cross only had a C++ API
and no guarantee of API or ABI stability, so instead of using
SPIRV-Cross directly, mpv used an unofficial C wrapper called crossc.
Now that KhronosGroup/SPIRV-Cross#611 is resolved, SPIRV-Cross has an
official C API that can be used instead, so remove crossc and use
SPIRV-Cross directly.