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/core/db/tests/test_db.py

@@ -255,7 +255,6 @@ def db_with_data(test_db, request):
     db.conn.close()
 
 
-@pytest.mark.db
 def test_db_connect(db_with_data):
     with db_with_data.cursor() as cur:
         cur.execute(INSERT_SQL, STATIC_ROW_IN)