delete some gdmwayland-on.sh environment variables to not break things under x11

• XDG_SESSION_TYPE should not be set globally or manually, this is sourced automatically at login and apps need it to be accurate to load correctly
• QT_QPA_PLATFORM is not necessary as Qt apps can figure out if they need xcb or wayland per-run
• QT_QPA_PLATFORMTHEME refers to qt5ct which is not packaged, and this does not seem necessary at all under GNOME 42
• MOZ_ENABLE_WAYLAND commented out as this causes graphics errors for Firefox under X11 and is not necessary to Firefox to function correctly under Wayland, but leave the option since the backend does work under Wayland

These changes fix Qt apps crashing when logged into Gnome on Xorg after enabling this option, allow Qt apps to use native (theme) titlebars under Wayland and X11, and prevent GNOME Tweaks, Extensions, and other apps getting confused and failing to work correctly under X11. Also prevents potential issues with Firefox.

See: my forum post here and this external guide for more details.