Stop using unifdef to generate bsdxml.h

The current invocation of unifdef causes the build to fail when using a shell
with -o pipefail on by default since unifdef will return a non-zero exit status
if it changes something. The only thing this call to unifdef does is remove 5
lines that will be ignored by the compiler anyway. Furthermore, it is the only
make rule in the source tree that requires unifdef. Removing this call also
makes it slightly easier to build without inhering $PATH (D16815) since we
don't need unifdef anymore.

I also noticed that the sed call to replace the include guard has been broken
for over 10 years since the import of expat 2.0.1 changed it from
XmlParse_INCLUDED to Expat_INCLUDED. I could also fix this but since it's
been broken for so long and no one noticed, it's probably not necessary.

Reviewed By: emaste
Approved By: jhb (mentor)
Differential Revision: https://reviews.freebsd.org/D14317