Drop some unused dependencies

* `ipython` was removed with 49b41a51
* `smmap2` looking at the history with `git log -G smmap --name-only
  --oneline` suggests this was only ever defined in
  `poetry.lock`/`pyproject.toml`. Given this is a library and not
  providing something extra like e.g. a `flake8` plugin it should be
  safe to remove