Revert "Replace PostmasterRandom() with a stronger way of generating randomness."

This reverts commit 9e083fd468. That was a
few bricks shy of a load:

* Query cancel stopped working
* Buildfarm member pademelon stopped working, because the box doesn't have
  /dev/urandom nor /dev/random.

This clearly needs some more discussion, and a quite different patch, so
revert for now.

src/include/port.h

@@ -454,9 +454,6 @@ extern int	pg_codepage_to_encoding(UINT cp);
 extern char *inet_net_ntop(int af, const void *src, int bits,
 			  char *dst, size_t size);
 
-/* port/pg_strong_random.c */
-extern bool pg_strong_random(void *buf, size_t len);
-
 /* port/pgcheckdir.c */
 extern int	pg_check_dir(const char *dir);