Remove configure switch --disable-strong-random

This removes a portion of infrastructure introduced by fe0a0b5 to allow
compilation of Postgres in environments where no strong random source is
available, meaning that there is no linking to OpenSSL and no
/dev/urandom (Windows having its own CryptoAPI).  No systems shipped
this century lack /dev/urandom, and the buildfarm is actually not
testing this switch at all, so just remove it.  This simplifies
particularly some backend code which included a fallback implementation
using shared memory, and removes a set of alternate regression output
files from pgcrypto.

Author: Michael Paquier
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/20181230063219.GG608@paquier.xyz

contrib/pgcrypto/px.c

@@ -56,7 +56,7 @@ static const struct error_desc px_err_list[] = {
 	{PXE_UNKNOWN_SALT_ALGO, "Unknown salt algorithm"},
 	{PXE_BAD_SALT_ROUNDS, "Incorrect number of rounds"},
 	{PXE_MCRYPT_INTERNAL, "mcrypt internal error"},
-	{PXE_NO_RANDOM, "No strong random source"},
+	{PXE_NO_RANDOM, "Failed to generate strong random bits"},
 	{PXE_DECRYPT_FAILED, "Decryption failed"},
 	{PXE_PGP_CORRUPT_DATA, "Wrong key or corrupt data"},
 	{PXE_PGP_CORRUPT_ARMOR, "Corrupt ascii-armor"},
@@ -97,17 +97,9 @@ px_THROW_ERROR(int err)
 {
 	if (err == PXE_NO_RANDOM)
 	{
-#ifdef HAVE_STRONG_RANDOM
 		ereport(ERROR,
 				(errcode(ERRCODE_INTERNAL_ERROR),
 				 errmsg("could not generate a random number")));
-#else
-		ereport(ERROR,
-				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-				 errmsg("generating random data is not supported by this build"),
-				 errdetail("This functionality requires a source of strong random numbers."),
-				 errhint("You need to rebuild PostgreSQL using --enable-strong-random.")));
-#endif
 	}
 	else
 	{