main/postmarketos-ui-i3wm|mate|xfce4: remove deinstall scripts

postmarketos-ui-i3wm|mate.post-deinstall:

• Contains removal of file "61-autologin.conf"|"65-autologin.conf". However, this file is removed by apk automatically unpon deinstallation.
• Reference: !2106 (comment 548853112)

postmarketos-ui-i3wm|mate|xfce4.pre-deinstall:

• Contains disabling service "lightdm". Removing display manager service upon deinstallation isn't done by other UIs either. It can lead to unexpected behaviour when installing another UI using lightdm and then uninstall MATE UI.
• Reference: !2106 (comment 564087640)

Additionally

postmarketos-ui-i3wm.post-upgrade:

• Removed postmarketos-ui-i3wm.post-upgrade and replaced it with a link to postmarketos-ui-i3wm.post-install, like it's done in MATE and Xfce4.