Remove forced default focus outline (twbs#20275)

As this causes non-standard dotted outline on checkboxes/radio buttons
in OS X Firefox

Ports twbs#19935 to v4
Refs twbs#19933

[skip sauce]

scss/mixins/_tab-focus.scss

@@ -1,9 +1,9 @@
 // WebKit-style focus
 
 @mixin tab-focus() {
-  // Default
-  outline: thin dotted;
-  // WebKit
+  // WebKit-specific. Other browsers will keep their default outline style.
+  // (Initially tried to also force default via `outline: initial`,
+  // but that seems to erroneously remove the outline in Firefox altogether.)
   outline: 5px auto -webkit-focus-ring-color;
   outline-offset: -2px;
 }