Remove deprecated box-shadow mixin from the Editor style.


@ejholmes
Bourbon produces a deprecation warning message when I using the ActiveAdmin Editor.
See gregbell /active_admin/pull/1913

    WARNING: box-shadow is deprecated and will be removed in the next major version release
        on line 7 of /app/vendor/bundle/ruby/2.0.0/gems/bourbon-3.1.8/app/assets/stylesheets/_bourbon-deprecated-upcoming.scss, in `box-shadow'
        from line 25 of /app/vendor/bundle/ruby/2.0.0/gems/active_admin_editor-1.1.0/app/assets/stylesheets/active_admin/editor.css.scss