Remove todo note about QXcbWindow::format()

It _is_ the actual format after create(). This is true at least
since modularizing the GLX and EGL integration. See resolveFormat()
in the integration plugins.

This is important to clarify since other platform plugins usually
return the input (requested) format as-is. We must therefore be
careful with misleading notes as the situation is complicated enough
already.

Change-Id: I8e394680fa8fe3c80941a2a08fa873f831d6403c
Reviewed-by: Andy Nichols <andy.nichols@theqtcompany.com>

src/plugins/platforms/xcb/qxcbwindow.cpp

@@ -1524,7 +1524,6 @@ void QXcbWindow::requestActivateWindow()
 
 QSurfaceFormat QXcbWindow::format() const
 {
-    // ### return actual format
     return m_format;
 }