Remove unused import in Quick Start Define Models

[huntertur]

Hello flask-sqlalchemy folks,

Thank you for updating the documentation to show off the 2.0 way of using SQLAlchemy. I've been re-reading through the Flask-SQLAlchemy documentation as part of upgrading a project that had originally been on SQLAlchemy 1.x, and learning about (and using) new things like sqlalchemy.orm.Mapped and sqlalchemy.orm.mapped_column has been very helpful.

This one-line PR removes an unused import statement from the documentation that I think may have been left over from the pre-mapped_column days. The PR template says to add a line to CHANGES.rst, but it looks like the existing pattern is to not update that file for minor documentation changes, so I did not touch that file.