Remove _colours.scss

- This only exists for the colour palette swatches
- Move the Sass list is to sit within the elements-page Sass file where
it is used to generate classnames for colour swatches
- All Sass variables for GOV.UK colours can be found in the
govuk_frontend_toolkit (
https://github.com/alphagov/govuk_frontend_toolkit/blob/master/styleshee
ts/_colours.scss)  there is no need to duplicate variable names in
govuk_elements