axisartist stores in .grid_info various intermediary calculations used
to draw gridlines (which may be curved) and ticks. It may be helpful to
change the internal representation of this info in the future (e.g.
using Paths directly rather than x, y arrays which have to be
continuously packed and unpacked), but doing so means that the attribute
should become private first.

Also remove the private _get_line_path, which was added in 008b765 but
apparently never ever used (we can always reintroduce something like
that if needed).

PR Summary

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).