Kill XSUN Xinerama support

XSun is a relic from Solaris 10 / SPARC.

Change-Id: I5c379a496ee5953d71fe51b86cf9992c92943126
Reviewed-on: https://gerrit.libreoffice.org/10756
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>