testing: remove a no longer necessary skip

pytest-xdist was released at 2019-02-15, seems enough time.

testing/test_config.py

@@ -1959,16 +1959,6 @@ def test_invocation_args(pytester: Pytester) -> None:
     ],
 )
 def test_config_blocked_default_plugins(pytester: Pytester, plugin: str) -> None:
-    if plugin == "debugging":
-        # Fixed in xdist (after 1.27.0).
-        # https://github.com/pytest-dev/pytest-xdist/pull/422
-        try:
-            import xdist  # noqa: F401
-        except ImportError:
-            pass
-        else:
-            pytest.skip("does not work with xdist currently")
-
     p = pytester.makepyfile("def test(): pass")
     result = pytester.runpytest(str(p), "-pno:%s" % plugin)