Remove Qt

The core runs only inside a libretro frontend. The Qt frontend and its
submodule go, together with what only it used: the Qt DBus screensaver
inhibit, the JIT's single-block erase, the string translator hook, the
Qt translation catalogs and Transifex config, the desktop packaging
(NSIS, RPM, Flatpak) and the autoupdate overview.

.gitmodules

@@ -1,7 +1,3 @@
-[submodule "Externals/Qt"]
-	path = Externals/Qt
-	url = https://github.com/dolphin-emu/ext-win-qt.git
-	shallow = true
 [submodule "Externals/mGBA/mgba"]
 	path = Externals/mGBA/mgba
 	url = https://github.com/mgba-emu/mgba.git