Doc: mention that Qt::AA_X11InitThreads isn't used anymore

Whether or not to call XInitThreads() doesn't apply since Qt-5.0's switch to XCB.

Change-Id: I5f1e5e664a251c98af6357c87fc9a6bb03a46ce3
Reviewed-by: Martin Smith <martin.smith@theqtcompany.com>
Reviewed-by: Topi Reiniö <topi.reinio@theqtcompany.com>

src/corelib/global/qnamespace.qdoc

@@ -144,9 +144,7 @@
            to be consistent in pixels-per-point across devices rather than
            defining 1 point as 1/72 inch.
 
-    \value AA_X11InitThreads Calls \c XInitThreads() as part of the QApplication
-           construction in order to make Xlib calls thread-safe. This
-           attribute must be set before QApplication is constructed.
+    \value AA_X11InitThreads This value is obsolete and has no effect.
 
     \value AA_SynthesizeTouchForUnhandledMouseEvents All mouse events
            that are not accepted by the application will be translated

tests/auto/opengl/qglthreads/tst_qglthreads.cpp

@@ -796,7 +796,6 @@ void tst_QGLThreads::painterOnFboInThread()
 
 int main(int argc, char **argv)
 {
-    QApplication::setAttribute(Qt::AA_X11InitThreads);
     QApplication app(argc, argv);
     QTEST_DISABLE_KEYPAD_NAVIGATION \