Disable session.remove on flask_sqlalchemy.SQLAlchemy

With Flask-SQLAlchemy 2.4.0, SQLAlchemy 1.3.3 and pytest-flask-sqlalchemy 1.0.2, I sometimes (non-deterministically) got `DetachedInstanceError` because the session instantiated in the `flask_sqlalchemy.SQLAlchemy` extension already terminated the session (https://github.com/pallets/flask-sqlalchemy/blob/master/flask_sqlalchemy/__init__.py#L849).

Adding something like this to `conftest.py` helped.

```
mocker.patch.object(sqla.session, "remove", lambda: None)
```