Do not declare asctime_r and ctime_r for C2X

ISO C2X added the asctime_r, ctime_r, gmtime_r and localtime_r
functions from POSIX.  It's now removed asctime_r and ctime_r again,
reflecting that they are marked obsolescent in POSIX; update glibc's
time.h accordingly.

The same change that removed those two functions from C2X also marked
asctime and ctime as deprecated (reflecting how POSIX shows them as
obsolescent), i.e. using the [[deprecated]] attribute in the
prototypes shown in C2X.  It's less clear if we should explicitly
deprecate those functions like that in the glibc headers; this patch
does nothing regarding such a deprecation (there's no normative
requirement from C2X showing the functions as deprecated).

Tested for x86_64 and x86.