Let Axes.clear iterate over Axises.

[anntzer]

... so that the implementation also works for 3D. This means also
adding a sharez() method, which is copy-pasted from sharex()/sharey()
(note that it is resolves a slight inconsistency in the old
Axes3D.clear, which would previously not copy the z-autoscaleness status
of a z-shared axes).

Using Axis._set_lim in the loop also means that we need to pass auto
explicitly to it anyways (it is a required parameter), so we can get rid
of the set_autoscalex/y/z calls.

Also, note that Axis.clear() explicitly resets the scale to "linear", so
the extra _set_scale("linear") calls are unneeded.

Finally, remove the silencing of TypeErrors when setting limits in
clear(), which was added in 917de33 to handle initialization order
problems in Axes3D but which seem not needed anymore (and are rather
unsightly, as it's not immediately clear what we're really catching).

PR Summary

PR Checklist

Tests and Styling

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• New features are documented, with examples if plot related.
• 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).
• Documentation is sphinx and numpydoc compliant (the docs should build without error).