Originally added with 78e8d5f0 "Added option to
hide external link buttons on Start Center".

The use of BackingWindow::mnHideExternalLinks was remove with
74144e53 "startcenter: Tweak Start Center
layout", and then the member and its initialization from the
StartCenterHideExternalLinks configuration property was removed with
d7824bf1 "loplugin:unusedfields in sfx2 part2".

(The opportunity to already remove STARTCENTER_HIDE_EXTERNAL_LINKS from
instsetoo_native/util/openoffice.lst.in had been missed in
89ac3c4a "replace variables in main.xcd already
in gbuild".)

Change-Id: I35bbd94db88939d7724616fa22a74e18552a4ad8
Reviewed-on: https://gerrit.libreoffice.org/55379Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>