The packages you need to remove are nvidia and nvidia-utils. If you get such dependency warnings, you can force the remove with pacman -Rd nvidia nvidia-utils. You can then install xf86-video-nouveau and nouveau-dri if you want.
Thanks alot! I never knew such command existed.
]]>archuser@archws ~ % sudo pacman -R nvidia-utils <19:13>
checking dependencies...
error: failed to prepare transaction (could not satisfy dependencies)
:: lib32-libgl: requires libgl
:: libva: requires libgl
:: mesa: requires libgl
:: mplayer: requires libgl
:: nvidia: requires nvidia-utils=260.19.36
:: qt: requires libgl
:: xorg-xdriinfo: requires libgl