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

include/stddef.h

@@ -41,13 +41,6 @@ typedef	__ptrdiff_t	ptrdiff_t;
 #define	_PTRDIFF_T_DECLARED
 #endif
 
-#if __BSD_VISIBLE
-#ifndef _RUNE_T_DECLARED
-typedef	__rune_t	rune_t;
-#define	_RUNE_T_DECLARED
-#endif
-#endif
-
 #ifndef _SIZE_T_DECLARED
 typedef	__size_t	size_t;
 #define	_SIZE_T_DECLARED

include/stdlib.h

@@ -38,13 +38,6 @@
 
 __NULLABILITY_PRAGMA_PUSH
 
-#if __BSD_VISIBLE
-#ifndef _RUNE_T_DECLARED
-typedef	__rune_t	rune_t;
-#define	_RUNE_T_DECLARED
-#endif
-#endif
-
 #ifndef _SIZE_T_DECLARED
 typedef	__size_t	size_t;
 #define	_SIZE_T_DECLARED