feat(CSS): remove gl-heading-scale SCSS mixin

mentioned in commit 2c1c461d

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!58 (merged)

mentioned in merge request status-page!903

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!4221 (merged)

mentioned in merge request gitlab!176018 (merged)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1407 (closed)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!359 (merged)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1446 (closed)