Remove attempt to support floating-point time_t.

It wasn't tested and probably never worked, no platform used it,
and the latest POSIX no longer allows it.
* Makefile (typecheck): Don't check time_t being 'double'.
* Theory: Document the change.
* difftime.c (difftime):
* localtime.c (differ_by_repeat, tzload, timesub, time2sub):
* private.h (time_t_min, time_t_max):
* zdump.c (absolute_min_time, absolute_max_time, tformat):
Don't try to support floating-point time_t.
* localtime.c, private.h, zdump.c: Don't include float.h.
* localtime.c (truncate_time, double_to_time):
* private.h (TYPE_INTEGRAL):
* zdump.c (checkabsolutes):
Remove; no longer needed.  All uses removed.
* zdump.8 (LIMITATIONS): Remove discussion of floating-point time_t.