Remove deprecated z coordinate in CrystalMap and related "3D functionality"

Description of the change

The appearance of "3D functionality" in the CrystalMap (implemented by me, but never tested on a real 3D dataset for its usefulness) was deprecated in 0.10.1 and will be removed in 0.11.0. This PR makes the necessary changes to the class, the plotting and IO plugins (orix HDF5 and ang).

The only issue I can see is that people have stored 3D datasets in orix' HDF5 format, which they now cannot read back with the changed orix HDF5 reader. I hope no-one has done this, but if they have, let's hope they shout out in an issue so we can fix this.

Progress of the PR

[x] Docstrings for all functions
[x] Unit tests with pytest for all lines
[x] Clean code style by running black via pre-commit

For reviewers

[ ] The PR title is short, concise, and will make sense 1 year later.
[ ] New functions are imported in corresponding __init__.py.
[ ] New features, API changes, and deprecations are mentioned in the unreleased section in CHANGELOG.rst.
[ ] Contributor(s) are listed correctly in __credits__ in orix/__init__.py and in .zenodo.json.

pyxem / orix