Remove non-working Jupyter magic.py

[rolandwalker]

Description

This file has not been touched substantively in a few years, is not documented, and no longer works.

If these missing dependencies are added to pyproject.toml:

• ipython-sql
• sql

Then the user can do the following in a notebook/console started from the mycli repo root:

In [1]: %load_ext sql
In [2]: %load_ext mycli.magic
In [3]: %sql mysql+pymysql://mycli:password@localhost:3306/mysql

However, upon executing a query with the magic

In [4]: %mycli show tables

we find that magic.py has

mycli: MyCli = conn._mycli

but mycli has long since been refactored, such that

AttributeError: 'Connection' object has no attribute '_mycli'

and it throws similarly for

 u = conn.session.engine.url

Given that this is not working, not maintained, and not documented, we should be comfortable deleting it without a deprecation cycle.

In the future, if this functionality is desired, it should probably be implemented in a separate library which takes mycli as a dependency. That way it could be more discoverable on PyPi, and could declare additional dependencies needed only in the notebook context.

Side note: this file was not covered by tests, and removing it brings the repo to 100% coverage.

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .