frontend: Remove V-Sync option from UI

The V-Sync option is fundamentally broken in Citra, so let's do the same as yuzu and remove it entirely for SDL2 and at least from the frontend for QT.
(It was also only used by 7.3% of users)

src/citra_qt/bootmanager.cpp

@@ -315,7 +315,7 @@ void GRenderWindow::InitRenderTarget() {
     QGLFormat fmt;
     fmt.setVersion(3, 3);
     fmt.setProfile(QGLFormat::CoreProfile);
-    fmt.setSwapInterval(Settings::values.use_vsync);
+    fmt.setSwapInterval(Settings::values.vsync_enabled);
 
     // Requests a forward-compatible context, which is required to get a 3.2+ context on OS X
     fmt.setOption(QGL::NoDeprecatedFunctions);