Dedupe RGBAxes examples.

[anntzer]

simple_rgb is essentially the second case of demo_axes_rgb, so remove
the former. Also small cleanups to demo_axes_rgb: get_demo_image can be
folded into get_rgb; demo_rgb2 doesn't need to return the axes.

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

[QuLogic]

I think there are some cross-references here:

/home/circleci/project/doc/tutorials/toolkits/axes_grid.rst:: WARNING: image file not readable: tutorials/toolkits/../../gallery/axes_grid1/images/sphx_glr_simple_rgb_001.png

[anntzer]

ok, swapped the order of the examples and moved the ticks-to-white to the second one (previously first one) to keep the "simple" case for display in the tutorial.