Remove pytest-xdist from tox now that it's in requirements_test.txt (#37455)

tox.ini

@@ -10,7 +10,6 @@ commands =
 deps =
      -r{toxinidir}/requirements_test_all.txt
      -c{toxinidir}/homeassistant/package_constraints.txt
-     pytest-xdist
 
 [testenv:cov]
 commands =
@@ -19,7 +18,6 @@ commands =
 deps =
      -r{toxinidir}/requirements_test_all.txt
      -c{toxinidir}/homeassistant/package_constraints.txt
-     pytest-xdist
 
 [testenv:pylint]
 ignore_errors = True