Drop mock_get_scheduler which creates indirection for no good reason

This is no longer useful, as removing it and tests are still ok.

Test Plan

tox

---

Migrated from D4287 (view on Phabricator)

swh/lister/tests/test_cli.py

@@ -19,7 +19,7 @@ def test_get_lister_wrong_input():
     assert "Invalid lister" in str(e.value)
 
 
-def test_get_lister(mock_get_scheduler):
+def test_get_lister():
     """Instantiating a supported lister should be ok
 
     """