Fixed double borders when using justified button groups

Issue: Double borders are being shown when using justified button groups (see examples at http://getbootstrap.com/components/#btn-groups-justified).

This can be resolved by setting `border-collapse: collapse` on the base class, but I noticed that it was explicitly set in 1e1b5c737ba33c0e7a86634bcb3baedc70498884 to resolve #9406.

I've fixed this by removing one of the borders when multiple buttons are present in a group.