Remove unnecessary dynamic_cast

OOXMLDocumentImpl is the only implementation of OOXMLDocument, though
code outside the ooxml directory has no access to OOXMLDocumentImpl.
However, in that directory passing OOXMLDocumentImpl* as OOXMLDocument*,
then using dynamic_cast to get back OOXMLDocumentImpl* is quite
pointless.

Change-Id: I06dcf85180def1db33b57bd356749f5d92aefa80