python3Packages.aiohttp: remove pytest-xdist

Fails reproducible on hydra like this:

```
        try:
            # set_wakeup_fd() raises ValueError if this is not the
            # main thread.  By calling it early we ensure that an
            # event loop running in another thread cannot add a signal
            # handler.
>           signal.set_wakeup_fd(self._csock.fileno())
E           ValueError: set_wakeup_fd only works in main thread of the main interpreter
```

And was remove from upstreams tests due to 10-15% error rate way back
in 2018.

pkgs/development/python-modules/aiohttp/default.nix

@@ -15,7 +15,6 @@
 , freezegun
 , gunicorn
 , pytest-mock
-, pytest-xdist
 , pytestCheckHook
 , re-assert
 , trustme
@@ -52,16 +51,11 @@ buildPythonPackage rec {
     freezegun
     gunicorn
     pytest-mock
-    pytest-xdist
     pytestCheckHook
     re-assert
     trustme
   ];
 
-  pytestFlagsArray = [
-    "-n auto"
-  ];
-
   disabledTests = [
     # Disable tests that require network access
     "test_mark_formdata_as_processed"