Revert "Implement Barrier"

This reverts commit 2e77be3.

Author: dandavison

tests/nexus/test_worker.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import asyncio
+import sys
 import uuid
 from datetime import timedelta
 from typing import Any
@@ -52,18 +53,10 @@ async def test_max_concurrent_nexus_tasks(
     if env.supports_time_skipping:
         pytest.skip("Nexus tests don't work with Javas test server")
 
-    class Barrier:
-        def __init__(self, size: int) -> None:
-            self.size = size
-            self.event = asyncio.Event()
+    if sys.version_info < (3, 11):
+        pytest.skip("Test requires Python 3.11+")
 
-        async def wait(self) -> None:
-            if len(self.event._waiters) >= self.size - 1:
-                self.event.set()
-            else:
-                await self.event.wait()
-
-    barrier = Barrier(num_nexus_operations)
+    barrier = asyncio.Barrier(num_nexus_operations)  # type: ignore
 
     @nexusrpc.handler.service_handler
     class MaxConcurrentTestService: