feat!: Drop the legacy Tabs UI (#37557)

This change drops the legacy studio custom pages UI aka. the tab edit
page.

This work is part of https://github.com/openedx/edx-platform/issues/36108

BREAKING CHANGE: The 'legacy_studio.custom_pages' waffle flag has been
removed and the code will work as if this flag is permanently set to
False.

Co-authored-by: Kyle McCormick <kyle@axim.org>

cms/static/js/models/explicit_url.js

@@ -1,14 +0,0 @@
-/**
- * A model that simply allows the update URL to be passed
- * in as an argument.
- */
-define(['backbone'], function(Backbone) {
-    return Backbone.Model.extend({
-        defaults: {
-            explicit_url: ''
-        },
-        url: function() {
-            return this.get('explicit_url');
-        }
-    });
-});