Fixes #14227 (closed) and #14603.
Due to ongoing problems with iOS rendering and unexpected clashes when folks are customizing Bootstrap, we're dropping the translate3d on modals and navbars. This PR leaves the ones in the carousel though for those smoother transitions (I don't think anyone's complained about those).
Only question I have: should we remove it from modals, or just navbars?
/cc @cvrebert @hnrch02