cleanup random.c

Casting Py_ssize_t to Py_ssize_t is useless.

Python/random.c

@@ -251,7 +251,7 @@ dev_urandom_noraise(unsigned char *buffer, Py_ssize_t size)
             break;
         }
         buffer += n;
-        size -= (Py_ssize_t)n;
+        size -= n;
     }
     close(fd);
 }