style: Remove the moz-focus-outer code.

This pseudo does nothing and we successfully removed it in bug 1655859
(firefox 81).

Differential Revision: https://phabricator.services.mozilla.com/D101289

components/style/gecko/pseudo_element.rs

@@ -161,9 +161,6 @@ impl PseudoElement {
     /// Whether this pseudo-element is enabled for all content.
     pub fn enabled_in_content(&self) -> bool {
         match *self {
-            PseudoElement::MozFocusOuter => {
-                static_prefs::pref!("layout.css.moz-focus-outer.enabled")
-            },
             PseudoElement::FileSelectorButton => {
                 static_prefs::pref!("layout.css.file-selector-button.enabled")
             },