Remove unneeded _update_transScale calls in _init_axis.

[anntzer]

_init_axis is called only in the Axes constructor, where it is followed by a call to clear, which itself already calls _update_transScale (which is needed to take into account the axis scales that get set in it). So _init_axis doesn't need to call _update_transScale itself. Removing it avoids showcasing a private method in the custom_projection demo, and helps moving towards a system where updating an axis scale directly invalidates whatever is necessary (e.g. via a TransformWrapper) instead of requiring manual invalidation calls (i.e. _update_transScale).

PR Summary

PR Checklist

Documentation and Tests

• Has pytest style unit tests (and pytest passes)
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• New plotting related features are documented with examples.

Release Notes

• New features are marked with a .. versionadded:: directive in the docstring and documented in doc/users/next_whats_new/
• API changes are marked with a .. versionchanged:: directive in the docstring and documented in doc/api/next_api_changes/
• Release notes conform with instructions in next_whats_new/README.rst or next_api_changes/README.rst

[dstansby]

👍 if tests pass