Don't bother with manually resizing the Qt main window.

[anntzer]

The presence of FigureCanvasQT.sizeHint is sufficient to get a correct
figure size; Qt properly determines the entire window size based on
that.

This behavior is already tested by test_backend_qt::test_dpi_ratio_change,
and one can also manually check that the patch does not change physical
window sizes when creating figures of various figsize=(?).

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[timhoffm]

QT is designed to get this right by itself. We need not and shouldn't try and explicitly calculate window sizes ourselves.