Show only source{d} palettes

[dpordomingo]

With #259 we added source{d} palettes, but we kept the ones offered by Apache Superset .

In order to let the user easily choose between our official palettes, it would be easier if the old superset palettes would not be there.

[marnovo]

@dpordomingo at this point it's more harm than good: we have to make a change and removing a degree of freedom from our users—which could for example using telemetry later on give cues that users dislike our palette or that they want to customize to their own brand colors, for example. If our palettes are already the default + top choices, there's little benefit to the cost.

[dpordomingo]

source{d} palettes are not the first ones, but added at the end of Apache Superset original ones.
Also, the users cannot add or customize their palettes, no matter we keep the original ones or not.