mate / xfce4: use tinydm, drop lightdm

I don't remember why these two aren't already using tinydm... if there ever was a reason. Anyways, they both start fine with tinydm.

fixes https://gitlab.com/postmarketOS/pmaports/-/issues/1960. Another way to fix this is to set the 'minimum-vt=1' option in lightdm.conf, to have lightdm run on VT1 instead of VT7. That sounds deceptively simple, however there are several packages in pmaports that install /etc/lightdm.conf, and if the user modified it then apk won't install the update with this option set. Edit ok I guess there's /usr/share/lightdm/lightdm.conf.d/, which should make adding this config fragment fairly easy/painless and could be used instead of this patch if we want to keep lightdm around. But I don't think there's a good reason to? Correct me if I'm wrong!