Remove dom.webcomponents.customelements.enabled pref.

This patch removes the following functions:

* nsContentUtils::IsCustomElementsEnabled()
* CustomElementRegistry::IsCustomElementEnabled(JSContext* aCx, JSObject* aObject)
* CustomElementRegistry::IsCustomElementEnabled(nsIDocument* aDoc)

and all references of the pref.

Depends on D11183

Differential Revision: https://phabricator.services.mozilla.com/D11249

components/style/gecko/selector_parser.rs

@@ -178,9 +178,6 @@ impl NonTSPseudoClass {
             NonTSPseudoClass::Fullscreen => unsafe {
                 mozilla::StaticPrefs_sVarCache_full_screen_api_unprefix_enabled
             },
-            NonTSPseudoClass::Defined => unsafe {
-                structs::nsContentUtils_sIsCustomElementsEnabled
-            },
             // Otherwise, a pseudo-class is enabled in content when it
             // doesn't have any enabled flag.
             _ => !self