Don't return NSXMLDTD objects

[williameveretteggplant]

Creating this PR for a patch. No need to merge this in.

GNUstep does not deal correctly with DTD nodes, which causes crashes. Just disable their creation until this is properly dealt with.

[rfm]

While I'm not really familiar with NSXMLNode (never having had cause to use it), this change makes no sense to me.
If, as the comment says, DTD nodes cause crashes, then please can we have some indication how/why things crash (testcases would be good) so that there is some chance of the code being fixed.
But also, if the comment is correct then why just remove two lines here and return nil? Presumably it would make more sense to comment out all the code creating DTD nodes, not just this one case.

[rfm]

I'm inclined to close this request since it appears to be an attempt to hide (instead of fix) a bug, and does not even provide a testcase (or clear statement of what conditions cause the bug) to allow the bug to be reproduced.