remove unused session_id test fixture

this was introduced in #275 for running the database in a container but is no longer needed

tests/conftest.py

@@ -247,12 +247,6 @@ def _register_table(table_name):
         loop.run_until_complete(cursor.execute(sql))
 
 
-@pytest.fixture(scope='session')
-def session_id():
-    """Unique session identifier, random string."""
-    return str(uuid.uuid4())
-
-
 @pytest.fixture(autouse=True)
 def ensure_mysql_version(request, mysql_image, mysql_tag):
     mysql_version = request.node.get_closest_marker('mysql_version')