Remove multiple navbar z-index callout

* Since we removed the z-index of our default navbars in 76baacf2c94bec03273639ef0ed966dad87cf37a, we no longer need this warning.
* A quick test of the new code showed no problems with dropdowns falling behind a sibling navbar, so folks should be good to go.
Merge branch 'master' of github.com:twbs/bootstrap
Fixes #11206: remove left padding on first inline list item
Merge branch 'master' of github.com:twbs/bootstrap