remove pointless coproc.h from arch/headless #3989

Job	Run time
Cleanup Previous Builds	4s
Create Draft Release	2s
Style check, Test Headless, Build Documentation	4m 54s
Build (MINGW64, x86_64, GTK3)	14m 11s
Build (MINGW64, x86_64, SDL2)	15m 13s
Build (MINGW64, x86_64, SDL1)	17m 41s
Build (MINGW32, i686, GTK3)	17m 12s
Build (MINGW32, i686, SDL2)	15m 25s
Build (MINGW32, i686, SDL1)	17m 7s
Build Debian Package (GTK3, --enable-gtk3ui --with-pulse, libglew-dev libgtk-3-dev libpulse-dev)	5m 40s
Build Debian Package (SDL2, --enable-sdl2ui --with-sdlsound --without-pulse, libsdl2-dev libsdl2-...	7m 41s
Build Debian Package (SDL1, --enable-sdl1ui --with-sdlsound --without-pulse, libsdl1.2-dev libsdl...	4m 6s
Build Debian Package (Headless, --enable-headlessui --with-pulse, libpulse-dev)	3m 49s
Publish Release	0s
IRC Notification of Build Errors	18s
	2h 3m 23s

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