Remove translate3d

Fixes #14227 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