missing_syscall: getrandom is supported by glibc since 2.25

Now our baseline of glibc is 2.31, hence it is OK to drop our
definition.

src/basic/missing_random.h

@@ -1,26 +1,11 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
+#include <sys/random.h>
+
 #include "macro.h"
 
-#if HAVE_GETRANDOM
-#  include <sys/random.h>
-#else
-#  include <linux/random.h>
-#endif
-
-#ifndef GRND_NONBLOCK
-#  define GRND_NONBLOCK 0x0001
-#else
-assert_cc(GRND_NONBLOCK == 0x0001);
-#endif
-
-#ifndef GRND_RANDOM
-#  define GRND_RANDOM 0x0002
-#else
-assert_cc(GRND_RANDOM == 0x0002);
-#endif
-
+/* Defined since glibc-2.32. */
 #ifndef GRND_INSECURE
 #  define GRND_INSECURE 0x0004
 #else