random: stop retrieving random bytes from OpenSSL

On the ::SLOW path we would use OpenSSL as an additional source of
random bytes. This commit removes that functionality. Note that this was
always only an additional source, and that we never checked the return
value

RAND_bytes(): https://www.openssl.org/docs/manmaster/man3/RAND_bytes.html

RAND_bytes() puts num cryptographically strong pseudo-random bytes into buf.

src/random.cpp

@@ -495,10 +495,6 @@ static void SeedSlow(CSHA512& hasher) noexcept
     GetOSRand(buffer);
     hasher.Write(buffer, sizeof(buffer));
 
-    // OpenSSL RNG (for now)
-    RAND_bytes(buffer, sizeof(buffer));
-    hasher.Write(buffer, sizeof(buffer));
-
     // High-precision timestamp.
     //
     // Note that we also commit to a timestamp in the Fast seeder, so we indirectly commit to a