Qt5: remove assert leading to crash while executing 'open' dialog

When Writer is opened using "qt5" VCL plugin,
opening menu item "File" -> "Open" leads to a crash
due to presence of mentioned assert.

Impress crashes same way when same menu item is used.

Change-Id: I9d1937344c694d4f4620be1b71d1181eb9645427
Reviewed-on: https://gerrit.libreoffice.org/65344
Tested-by: Jenkins
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>

vcl/qt5/Qt5FilePicker.cxx

@@ -187,7 +187,6 @@ sal_Int16 SAL_CALL Qt5FilePicker::execute()
     }
 
     vcl::Window* pWindow = ::Application::GetActiveTopWindow();
-    assert(pWindow);
     QWidget* pTransientParent = nullptr;
     QWindow* pTransientWindow = nullptr;
     if (pWindow)