Remove some unnecessary getattrs.

[anntzer]

Figures always have a canvas attached (even if it's just a
FigureCanvasBase), and canvases always have a "manager" attribute (which
may be None).

Canvases also always have a "toolbar" attribute (which also may be
None).

Altogether this allows avoiding a couple of getattrs.

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).