Don't bother with uintmax_t in previous change.

See the thread starting with Clive D.W. Feather's comments in
<http://mm.icann.org/pipermail/tz/2013-August/019496.html>.
* localtime.c (truncate_time): Don't bother with uintmax_t,
as using it doesn't help on any known platform.
* private.h (UINTMAX_MAX): Remove.