Removed unused leave-settings modal

refs https://github.com/TryGhost/Team/issues/1734
refs https://github.com/TryGhost/Team/issues/559

- all uses of the old-style modal have been removed so it's no longer needed

ghost/admin/app/components/modal-leave-settings.hbs

@@ -1,17 +0,0 @@
-<header class="modal-header" data-test-modal="unsaved-settings">
-    <h1>Are you sure you want to leave this page?</h1>
-</header>
-<a class="close" href="" role="button" title="Close" {{action "closeModal"}}>{{svg-jar "close"}}<span class="hidden">Close</span></a>
-
-<div class="modal-body">
-    <p>
-        Hey there! It looks like you didn't save the changes you made.
-    </p>
-
-    <p>Save before you go!</p>
-</div>
-
-<div class="modal-footer">
-    <button class="gh-btn" data-test-stay-button type="button" {{action "closeModal"}}><span>Stay</span></button>
-    <button class="gh-btn gh-btn-red" data-test-leave-button type="button" {{action "confirm"}}><span>Leave</span></button>
-</div>

ghost/admin/app/components/modal-leave-settings.js

@@ -1,14 +0,0 @@
-import ModalComponent from 'ghost-admin/components/modal-base';
-import RSVP from 'rsvp';
-
-export default ModalComponent.extend({
-    actions: {
-        confirm() {
-            this.confirm();
-            this.send('closeModal');
-        }
-    },
-
-    // Allowed actions
-    confirm: () => RSVP.resolve()
-});