refactor: Remove unused close_session method from SessionMixin

Author: daireto

sqlactive/session.py

@@ -31,11 +31,6 @@ def set_session(cls, session: async_scoped_session[SQLAlchemyAsyncSession]) -> N
         """
         cls._session = session
 
-    @classmethod
-    def close_session(cls) -> None:
-        """Close the async session."""
-        cls._session = None
-
     @classproperty
     def AsyncSession(cls) -> async_scoped_session[SQLAlchemyAsyncSession]:  # noqa: N802
         """Async session factory.