Remove unused org colour classes

[AshGDS]

What / Why

• Removes a couple org colours from our _brand-colours SASS file.
• The ministry-of-housing-communities-and-local-government class existed as a way to add the brand colour before govuk-frontend added it
• We added department-for-business-and-trade as the class in govuk-frontend was renamed to department-for-business-trade, so to stop the brand colours disappearing when govuk-frontend was deployed, we added them here.
• These organisations do not use these class names anymore as well, so they're safe to remove from the gem: https://github.com/alphagov/whitehall/pull/9379/files

Visual Changes

None.