remove unnecessary sal_Unicode casts in XMLOFF module

Change-Id: I836c7a28d5ec9039c0b185e8db2aa6d01ac62566
Reviewed-on: https://gerrit.libreoffice.org/6665
Tested-by: LibreOffice gerrit bot <gerrit@libreoffice.org>
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>