x11: Don't force QT_QPA_PLATFORM=xcb

This is basically a cherry pick of 77ec43d5e (Don't force
QT_QPA_PLATFORM=wayland), except for X11.

src/main_x11.cpp

@@ -362,6 +362,8 @@ int main(int argc, char *argv[])
 
     KWin::ApplicationX11 a(argc, argv);
     a.setupTranslator();
+    // reset QT_QPA_PLATFORM so we don't propagate it to our children (e.g. apps launched from the overview effect)
+    qunsetenv("QT_QPA_PLATFORM");
 
     KSignalHandler::self()->watchSignal(SIGTERM);
     KSignalHandler::self()->watchSignal(SIGINT);