chore: swap vf-core dev theme to mandlebrot

[khawkins98]

Following conversations in #1260 and #1261, we don't really need a custom fractal theme anymore. This swaps back to the default theme for vf-core development.

It also removes the vf-frctl-theme (which could still be resurrected from npm, if needed)

[sturobson]

[sturobson]

tried this - and I'm still getting the custom theme - with it error'ing when clicking on a component.

Doing this in fractal.js fixed it:

// var vfThemePath = global.vfThemePath || '@frctl/mandelbrot';
const vfTheme = require('@frctl/mandelbrot');

is my

git fetch origin
git reset --hard origin/develop

along with sprinkles of git clean fed beforehand not working properly?