changed OUStringBuffer::append() to op+. deleted unnecessary variable.

Change-Id: Ibe27f7f587f7130e7c31e33a29a88c5ea6478740
Reviewed-on: https://gerrit.libreoffice.org/43395Reviewed-by: Julien Nabet <serval2412@yahoo.fr>
Tested-by: Jenkins <ci@libreoffice.org>