tests: Remove no longer needed pytest custom marker named db

This was used at the time we were building debian packages for
swh components but we no longer do that.

swh/lister/rubygems/tests/test_lister.py

@@ -133,7 +133,6 @@ def network_requests_mock(datadir, requests_mock):
     )
 
 
-@pytest.mark.db
 def test_rubygems_lister(swh_scheduler, expected_listed_origins):
     lister = RubyGemsLister(scheduler=swh_scheduler)
     res = lister.run()