Removes double border in btn-group-justified.

A solution for #11964.

Tested with multiple button styles and dropdowns. Definitely adds some css, but
not a terribly unreasonable amount.

Conflicts:
	dist/css/bootstrap.css.map
	dist/css/bootstrap.min.css
	docs/assets/js/customize.min.js
	docs/assets/js/raw-files.min.js
	docs/dist/css/bootstrap.css.map
	docs/dist/css/bootstrap.min.css