don't generate invalid xml files

VmlCommentExporter::StartShape already adds a XML_type to the attribute
list, so we would be exporting the same attribute twice which makes the
xml file invalid. This file could therefore not be imported again.

Change-Id: I2aed2805ea59c15b1f7e60ab9cab10f7a8e81412
Reviewed-on: https://gerrit.libreoffice.org/6980
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>