Remove AbstractWindow.bg_color trait

[aaronayres35]

closes #812

This PR simply removes the old deprecated trait. Note searching the codebase for bg_color is what motivated issue #815. Perhaps enable/trait_defs/ui/wx/enable_rgba_color_editor.py should just be deleted? given 815 I imagine it is completely broken

[rahulporuri]

LGTM. I think we can remove this outright without worrying about breaking any code. Can you also search chaco for uses of bg_color, just to be safe?

Also, I think we can remove the enable_rgba_color_editor.py module but let's do that in a separate PR

[aaronayres35]

Also, I think we can remove the enable_rgba_color_editor.py module but let's do that in a separate PR

👍 I've opened #817

[aaronayres35]

LGTM. I think we can remove this outright without worrying about breaking any code. Can you also search chaco for uses of bg_color, just to be safe?

After searching chaco for bg_color there are a few uses, but the majority of which bg_color is used as a temp variable that is passed to bgcolor. eg bgcolor=bg_color.
There are 2 problem uses tho:
https://github.com/enthought/chaco/blob/54a1b929d3e3932bc79e17de5995658bd30e1b37/examples/demo/basic/contour_cmap_plot.py#L65

https://github.com/enthought/chaco/blob/54a1b929d3e3932bc79e17de5995658bd30e1b37/examples/demo/basic/contour_plot.py#L60
All uses occur in example code, not in library code.

I will open a PR to address these in chaco
EDIT: I've opened enthought/chaco#742