epubcheck complains:

ERROR(HTM-003): test.epub/OEBPS/images/image0001.svg(5675,37): External entities are not allowed in EPUB v3 documents. External entity declaration found: %svg-extensibility.mod.

and similar ones. Just not writing the DTD header is enough to address
the error.

Change-Id: I5307e932a0f07585297cce734aceae77e43cc7a6
Reviewed-on: https://gerrit.libreoffice.org/45648Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>