Remove xmlparse(document '') test

Remove xmlparse(document '') test

This one test was behaving differently between the ubuntu fix for
CVE-2015-7499 and the base "expected" file.  It's not worth having
yet another version of the expected file for this test, so drop it.
Perhaps at some point when all distros have settled down to the
same behavior on this test, it can be restored.

Problem found by me on libxml2 (2.9.1+dfsg1-3ubuntu4.6).
Solution suggested by Tom Lane.
Backpatch to 9.5, where the test was added.