stddef.h/stdlib.h: Remove unused rune_t

This non-standard type is unused in the base system (ct_rune_t or
rune_t are used instead) and ports. It has been around as long as our
current source repo, but we have avoided using it. In sys/_types.h
where the __*rune_t typedefs are defined, the following appears in a
comment:

NOTE: rune_t is not covered by ANSI nor other standards, and should
not be instantiated outside of lib/libc/locale.  Use wchar_t.

The definition of this unused type meant we gratutiously differed from
standards compliant stddef.h/stdlib.h.

PR: 279357 (exp-run by antoine)

Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D45426