Remove text-color prop from Label component

The Label component currently requires both a background color and a text color. Only background color should be required. Text color should be calculated based on that.

For example, this will give a black label with black text. Nobody wants this.

<gl-label
  background-color="#000"
  color="dark"
/>

Current label text function: https://gitlab.com/gitlab-org/gitlab/blob/master/app/helpers/labels_helper.rb#L113