libc: Remove incorrectly defined __STDC_VERSION_STDBOOL_H__

C23 does not define __STDC_VERSION_STDBOOL_H__ for <stdbool.h>.
Feature test macros of this form only apply to headers where the
standard explicitly mandates one.

Reviewed by:	fuz
Approved by:	fuz (mentor)
MFC after:	1 month
Differential Revision:	https://reviews.freebsd.org/D59136

include/stdbool.h

@@ -26,9 +26,6 @@
  * SUCH DAMAGE.
  */
 
-#ifndef __STDC_VERSION_STDBOOL_H__
-#define __STDC_VERSION_STDBOOL_H__ 202311L
-
 #ifndef __bool_true_false_are_defined
 #define	__bool_true_false_are_defined	1
 
@@ -41,4 +38,3 @@
 
 #endif /* (__STDC_VERSION__ < 202311L) && !defined(__cplusplus) */
 #endif /* __bool_true_false_are_defined */
-#endif /* __STDC_VERSION_STDBOOL_H__ */