Remove nv-codec-headers and stop building NVDEC

- NVDEC is disabled by default in Moonlight Qt now
- NVDEC is superseded by Vulkan Video
- NVDEC is only built into the desktop packages which we don't publish

.gitmodules

@@ -1,9 +1,6 @@
 [submodule "FFmpeg"]
 	path = FFmpeg
 	url = https://github.com/cgutman/FFmpeg.git
-[submodule "nv-codec-headers"]
-	path = nv-codec-headers
-	url = https://git.videolan.org/git/ffmpeg/nv-codec-headers.git
 [submodule "SDL2"]
 	path = SDL2
 	url = https://github.com/libsdl-org/SDL.git