Remove vestigial and inappropriate isnan()/isinf() timestamp tests.

Remove vestigial and inappropriate isnan()/isinf() timestamp tests.

I'm fairly sure this is an historical accident, but regardless, remove
the (my) obviously inappropriate floating point tests on integer
values from bup_parse_xutime_args().  Nothing to see here -- move
along.

I've broken this change out from a larger post-0.25 patch, where I'd
already fixed it, so that it'll be easy to pull into stable if we
like.

Thanks to Ed Maste <carpeddiem@gmail.com> for reporting the problem
(which prompted me to go ahead and create the separate fix).

Signed-off-by: Rob Browning <rlb@defaultvalue.org>