Fix bug where a panel recieve extra top border

https://github.com/twbs/bootstrap/issues/13734

A panel placed inside of a collapsible panel recieve extra top border.

This commit reduces the effect of a selector to apply a border only to
the collapsible panel body itself, not to any panel placed within it.
Merge pull request #13735 from skhilko/master

Fix bug where a panel recieve extra top border when placed within a collapsible panel