pki: Remove unnecessary and problematic chunk_from_chars() usage in --signcrl

If the serial is not yet set, the same default value is set just below.

See 8ea13bbc5c for details on chunk_from_chars().

References #3249.

src/pki/commands/signcrl.c

@@ -385,10 +385,6 @@ static int sign_crl()
 	}
 	else
 	{
-		if (!crl_serial.ptr)
-		{
-			crl_serial = chunk_from_chars(0x00);
-		}
 		lastenum = enumerator_create_empty();
 	}