Prevent edge case that creates extra event loop

`asyncio.get_event_loop_policy().get_event_loop() is asyncio.get_running_loop()` is not always true, as mentioned in python/cpython#96377 (comment). The AsyncioSelectorReactor runs in the second thread and uses the already initialised event loop in the main thread to run the crawler, so a single event loop will be running code from two threads.

scrapy/utils/reactor.py

@@ -90,7 +90,6 @@ def _get_asyncio_event_loop():
 
 def set_asyncio_event_loop(event_loop_path):
     """Sets and returns the event loop with specified import path."""
-    policy = get_asyncio_event_loop_policy()
     if event_loop_path is not None:
         event_loop_class = load_object(event_loop_path)
         event_loop = event_loop_class()
@@ -109,15 +108,13 @@ def set_asyncio_event_loop(event_loop_path):
                     message="There is no current event loop",
                     category=DeprecationWarning,
                 )
-                event_loop = policy.get_event_loop()
+                event_loop = asyncio.get_event_loop()
         except RuntimeError:
             # `get_event_loop` raises RuntimeError when called with no asyncio
             # event loop yet installed in the following scenarios:
-            # - From a thread other than the main thread. For example, when
-            #   using ``scrapy shell``.
             # - Previsibly on Python 3.14 and later.
             #   https://github.com/python/cpython/issues/100160#issuecomment-1345581902
-            event_loop = policy.new_event_loop()
+            event_loop = asyncio.new_event_loop()
             asyncio.set_event_loop(event_loop)
     return event_loop