fdo#72397: vcl: do not enable A11y in Win32 message handler

On the libreoffice-4-2 branch, IA2 is experimental so it must be enabled
by user explicitly; the automatic enabling when a WM_GETOBJECT is
received will effectively enable the JAB which can be annoying.

Change-Id: I16bd8d77657567ad29f3538cd3ab1de8145d531e
Reviewed-on: https://gerrit.libreoffice.org/7443
Reviewed-by: Eike Rathke <erack@redhat.com>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>