Remove redundant mixins

CHANGELOG

app/assets/stylesheets/framework/avatar.scss

app/assets/stylesheets/framework/blocks.scss

app/assets/stylesheets/framework/buttons.scss

app/assets/stylesheets/framework/forms.scss

app/assets/stylesheets/framework/issue_box.scss

app/assets/stylesheets/framework/markdown_area.scss

app/assets/stylesheets/framework/mixins.scss

app/assets/stylesheets/framework/mobile.scss

app/assets/stylesheets/framework/selects.scss

app/assets/stylesheets/framework/sidebar.scss

app/assets/stylesheets/framework/typography.scss

app/assets/stylesheets/pages/cycle_analytics.scss

app/assets/stylesheets/pages/editor.scss

app/assets/stylesheets/pages/events.scss

app/assets/stylesheets/pages/labels.scss

app/assets/stylesheets/pages/login.scss

app/assets/stylesheets/pages/merge_requests.scss

app/assets/stylesheets/pages/notes.scss

app/assets/stylesheets/pages/profiles/preferences.scss