Drop Qt style overrides

[danirabbit]

Fixes #286
Fixes #298
Closes #304

[bobby285271]

Hmmm I actually still kept "qt5-gtk-platformtheme" (we don't split packages so libqgtk3.so is actually provided by qt5.qtbase/qt6.qtbase) and QT_QPA_PLATFORMTHEME=gtk3 in NixOS Pantheon because Qt does not likely know Pantheon and I have the impression this can probably affect the use of native GTK dialogs, but yeah we don't have qgnomeplatform or adwaita-qt pre-installed now. I don't know how qgnomeplatform is used in elementary OS, but I think FedoraQt/QGnomePlatform@d86d6ba will make it stop using the gtk3 key 🤔

[danirabbit]

We don't ship anything Qt and as far as I can tell, Qt Flatpak apps aren't using any kind of Gtk style so I don't think this is ever used. I think these days we expect that apps use the FileChooser Portal 🤷‍♀️

[TomiOhl]

One relatively widely-used application that is affected by this setting is Telegram desktop (official non-flatpak build). See a related issue: telegramdesktop/tdesktop#26370
What I did to try to test this:

• unset QT_QPA_PLATFORMTHEME, unset QT_STYLE_OVERRIDE
• launch tdesktop with the system set to both light and dark mode
• switched light/dark mode while the app is running
• opened the file chooser

Everything seems to look fine and work as before, so as long as unsetting that env is equivalent to testing the changes in this PR, tdesktop won't break 🎉
(QT_STYLE_OVERRIDE=adwaita was an invalid value anyways according to tdesktop's logs)

[danirabbit]

@bobby285271 any further thoughts on this? or requested changes?

[bobby285271]

Nope, we have some NixOS-specific way to declare session environment variables and I don't think I can test this PR on NixOS, otherwise I have no objections :-)