Remove pytest fixture for event loop for session

It's cleaner to run an event loop for each test rather
than for the entire session specially when the gRPC client
is a singleton and its internal state can be corrupted
when used concurrently from multiple async contexts.

Author: daniel-zullo-frequenz

integration_tests/test_api.py

@@ -599,11 +599,3 @@ async def test_multiple_streams_different_filters(set_up: dict[str, Any]) -> Non
         ), "Streamed order does not match area-specific order in stream 2"
     except asyncio.TimeoutError:
         pytest.fail("Failed to receive streamed orders within timeout")
-
-
-@pytest.fixture(scope="session")
-def event_loop() -> Generator[asyncio.AbstractEventLoop, None, None]:
-    """Create an event loop for the tests."""
-    loop = asyncio.get_event_loop_policy().new_event_loop()
-    yield loop
-    loop.close()