The post editor helpfully removes default arguments like
columns="3" from the
[gallery] shortcode. How do you tell it to remove a custom attribute that has a default value?
The answer at Enhance Media Manager for Gallery shows how to add a custom attribute to the Wordpress 4.0 media manager. That code extends the backbone stuff in
wp-includes/js/media-views.js. The shortcode is actually constructed in
wp-admin/js/gallery.js, a tinyMCE plugin.
It's not purely an aesthetic goal to remove the default attribute. It would also help my plugin leave behind less cruft in the posts.