random: getentropy on macOS does not need unistd.h

Remove it. Make this change, so in a future commit, we can
combine #ifdefs, and avoid duplicate <sys/random.h> includes once we
switch to using getrandom directly.

Also remove the comment about macOS 10.12. We already require macOS >
10.15, so it is redundant.

src/random.cpp

@@ -33,7 +33,6 @@
 #include <linux/random.h>
 #endif
 #if defined(HAVE_GETENTROPY_RAND) && defined(MAC_OSX)
-#include <unistd.h>
 #include <sys/random.h>
 #endif
 #ifdef HAVE_SYSCTL_ARND
@@ -314,8 +313,6 @@ void GetOSRand(unsigned char *ent32)
     // Silence a compiler warning about unused function.
     (void)GetDevURandom;
 #elif defined(HAVE_GETENTROPY_RAND) && defined(MAC_OSX)
-    /* getentropy() is available on macOS 10.12 and later.
-     */
     if (getentropy(ent32, NUM_OS_RANDOM_BYTES) != 0) {
         RandFailure();
     }