docs: remove depreciated system_packages option from rtd config #4459

Job	Run time
GitHub Env Debug	2s
Check Changelog	5s
Setup Flatpak Matrix	3s
Setup Release	1s
Check Versions	0s
Linux (appimage, -DSUNSHINE_CONFIGURE_APPIMAGE=ON, 20.04)	13m 54s
Macports	11m 31s
Linux Flatpak (x86_64)	19m 5s
MacOS	8m 38s
Windows	11m 36s
Release to WinGet	0s
	1h 4m 55s

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