Prefer RTF when we have both RICHTEXT and RTF.

(cherry picked from commit dba3cd50)

Change-Id: Ib4133ae4fdecc32429d89b56b0c9466dd3451522
Reviewed-on: https://gerrit.libreoffice.org/47339Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>