Explicitly setting a particular outline (e.g. the default outline: thin dotted) overrides heuristics in some modern browsers (such as Firefox) that automagically suppress outline as a result of mouse/tap interaction (while retaining it for keyboard interaction).
This removes the generic tab-focus mixin (mostly leaving outline handling up to the browser and its own heuristics), but also adds explicit outline: none in cases where it's deemed "safe", i.e. where sufficiently clear focus indication is provided (.btn elements, pagination links)
Fixes https://github.com/twbs/bootstrap/issues/18650 / https://github.com/twbs/bootstrap/issues/17006 and the Firefox focus part of https://github.com/twbs/bootstrap/issues/19398