Revert "should_exit is not actually used by http". It was being used …

…by uvicorn and I hadn't noticed

This reverts commit 3ca8aec.

python/cog/server/http.py

@@ -415,6 +415,7 @@ def start(self) -> None:
 
     def stop(self) -> None:
         log.info("stopping server")
+        self.should_exit = True
 
         self._thread.join(timeout=5)
         if not self._thread.is_alive():