Delete all support for Qt

Our Qt backend has long been left behind and doesn't even support Vulkan
currently. There would be a lot of work to make it viable, and I don't
think anyone is really interested.

ImGui on SDL will soon fulfill the need for a more classic user interface
with a menu bar on Linux, and on Mac we already have a native UI.

AGENTS.md

@@ -55,7 +55,6 @@ Windows/main.cpp
 ios/main.cpp
 SDL/SDLMain.cpp
 UWP/PPSSPP_UWPMain.cpp
-Qt/main.cpp
 android/jni/app-android.cpp
 libretro/libretro.cpp