No longer call draw_if_interactive in parasite_axes.

[anntzer]

This is a remnant of the pre-staleness system, where all pyplot
functions used to call draw_if_interactive to trigger a draw_idle; this
has now been superseded with the stale system which likewise triggers a
draw_idle (here via add_subplot, which marks the figure as stale).

This way, the only remaining places that call draw_if_interactive
internally are in plt.figure() and in figure unpickling.

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).