Remove mixins not listed in typography styleguide
Remove mixins not listed in typography styleguide

Mixins should only include the font sizes specified in
http://alphagov.github.com/design/gov.uk.typography/ so removing all
others.

kalleth/static,robinwhittleton/static,robinwhittleton/static,kalleth/static,tadast/static,tadast/static,alphagov/static,alphagov/static,tadast/static,kalleth/static,alphagov/static,tadast/static,robinwhittleton/static,robinwhittleton/static,kalleth/static