Remove unused Window parameter from SystemSettingsChanging

This took a bit of code archaelogy for me to track down. It turns out that
as part of an effort to optimize startup, the line of code in this function
was commented out, presumably to be rewritten later. This happened in
commit ee3351d78c in July 2001 (!).

About three years later, in February 2004, the function was rewritten in
commit 189c2388d80. As it turns out, the only two functions that used
the Window parameter were vcl/source/window/window.cxx (which passed
itself in, but of course this did nothing) and
vcl/source/window/winproc.cxx. Furthermore, winproc only ever passed in
the first frame, so it didn't really do anything either.

Consequently, the function as it stands *now* only notifies the
application that system settings have been changed. It doesn't care
which window it tells.

Therefore, I have excised this parameter from the function. I don't
think it made sense when it was implemented anyway, so there is no net
loss.

After removing the unneeded parameter, I was also able to remove the
Window parameter from winproc.cxx's ImplHandleSalSettings function as
it was only ever used to set the top level window, which is now
irrelevant.

Change-Id: I84f2c5c5ff8969387da3af81e4a9c7f9ac6237e1
Reviewed-on: https://gerrit.libreoffice.org/7541
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>