Do not apply a mark to async_http_client fixture (#1700)

Applying marks to a pytest fixture has never actually worked, it has
just been a no-op. Starting with pytest 9, it now raises an error to
fix code that does so. Remove the mark.

Author: s-t-e-v-e-n-k

tests/test_integration.py

@@ -335,7 +335,6 @@ def do_request(self, req_num):
         usage = telemetry["last_request_metrics"]["usage"]
         assert usage == ["stripe_client", "async"]
 
-    @pytest.mark.anyio
     @pytest.fixture(params=["aiohttp", "httpx"])
     async def async_http_client(self, request, anyio_backend):
         if request.param == "httpx":