Simplify _redo_transform_rel_fig.

[anntzer]

• width_ratios and height_ratios are never None (None gets normalized to
  [1, 1, ...] in the setters).
• We don't need to separately handle the is_first_row()/is_last_col()
  cases (the sums will evaluate to 0/1 as needed).
• Naming the width/height dx/dy makes everything line up neatly.

PR Summary

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• 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).