Moving to black from defaults channel

I think we should deprecate our own black recipe in favour of that on the defaults channel (conda search black) shows me 19.3b0 is available there.

If you agree, we should just remove the conda/black directory in this package.

I'm not sure it is also good to remove our own compiled versions of black from our channel, since that is not part of any package's runtime.

@samuel.gaist: please let me know of your thoughts on this.