Merge SDL_wasapi_win32 into SDL_wasapi

[Lzard]

Moves code from SDL_wasapi_win32.c into SDL_wasapi.c, following support removal of WinRT.

https://github.com/libsdl-org/SDL/blob/8a2cac7aaade086fce7c8e645fca606371da13d7/src/audio/wasapi/SDL_wasapi_win32.c#L22-L29

[icculus]

This is something I wanted to do but didn't have time to clean up, so I'm 100% okay with this PR.

Let's definitely decide we aren't reverting #10724 before we merge this, but otherwise I approve.

[slouken]

Merged, thanks!