old color schemes lingering

[gordonwoodhull]

This is a follow-up to #1403.

We were focused on removing schemeCategory20c, because it's bad, but we ended up introducing a color defaulting system which is more general.

This Stack Overflow question points out that the coordinate grid mixin will still default to schemeCategory10, which is less bad but not what the user may expect if they set dc.config.defaultColors():

dc.js/src/coordinate-grid-mixin.js

Line 24 in 6a638e3

_chart.colors(d3.scaleOrdinal(d3.schemeCategory10));

Seems like we should remove that - I guess the purpose of this line was to opt out of 20-category and use 10-category, because the 20-category color scheme is bad. So it's redundant now.

Also, searching the sources for "scheme", I noticed that the color mixin still references schemeCategory20c in the documentation, when it should refer to dc.config.defaultColors() instead:

dc.js/src/color-mixin.js

Line 33 in 6a638e3

* @param {d3.scale} [colorScale=d3.scaleOrdinal(d3.schemeCategory20c)]