macro: Use C11 noreturn only

No need to provide a fallback as we compile with gnu11.

src/fundamental/macro-fundamental.h

@@ -24,15 +24,7 @@
 #else
 #define _fallthrough_
 #endif
-/* Define C11 noreturn without <stdnoreturn.h> and even on older gcc
- * compiler versions */
-#ifndef _noreturn_
-#if __STDC_VERSION__ >= 201112L
 #define _noreturn_ _Noreturn
-#else
-#define _noreturn_ __attribute__((__noreturn__))
-#endif
-#endif
 
 #define XSTRINGIFY(x) #x
 #define STRINGIFY(x) XSTRINGIFY(x)