Drop mock_get_scheduler which creates indirection for no good reason

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

conftest.py

@@ -5,24 +5,6 @@
 
 import os
 
-import pytest
-
 pytest_plugins = ["swh.scheduler.pytest_plugin", "swh.lister.pytest_plugin"]
 
 os.environ["LC_ALL"] = "C.UTF-8"
-
-
-@pytest.fixture
-def mock_get_scheduler(monkeypatch, swh_scheduler):
-    """Override the get_scheduler function in swh.lister.core.lister_base, to
-    return the swh_scheduler fixture.
-    """
-    from swh.lister.core import lister_base
-
-    # Match the signature from swh.scheduler.get_scheduler
-    def get_scheduler(cls, args={}):
-        return swh_scheduler
-
-    monkeypatch.setattr(lister_base, "get_scheduler", get_scheduler)
-
-    yield monkeypatch