qt: Drop extra include from vcl/qt6/QtMenu.cxx

This is already included in vcl/inc/qt5/QtMenu.hxx
since

    commit 50a7ed2138
    Author: Gabor Kelemen <gabor.kelemen@collabora.com>
    Date:   Tue Jan 6 11:53:54 2026 +0100

        Fix some non self contained headers in module vcl

(and would otherwise make more sense directly in
vcl/qt5/QtMenu.cxx instead of the "Qt 6 wrapper", to keep
the idea that the logic is in the source files shared by
both Qt versions, in particular given the include makes
sense for both versions).

This prepares for upcoming commit

    Change-Id: Ia52f5cfb8360e3b0de19404879d59131c63756b0
    Author: Michael Weghorn <m.weghorn@posteo.de>
    Date:   Thu Mar 26 10:59:25 2026 +0100

        qt: Include <sal/config.h>, arrange includes more consistently

to actually have the desired effect to avoid build
problems with the qt6 vcl plugin in experiments on Windows.

Change-Id: Ib62052a097881328a0dd6356d9483cc94291793e
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/202727
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>
Tested-by: Jenkins

vcl/qt6/QtMenu.cxx

@@ -7,8 +7,6 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/.
  */
 
-#include <QtWidgets/QMenu>
-
 #include "../qt5/QtMenu.cxx"
 
 /* vim:set shiftwidth=4 softtabstop=4 expandtab cinoptions=b1,g0,N-s cinkeys+=0=break: */