Remove the 'notification' feature flag

This is deployed and toggled on everywhere, so we can simplify our code
by removing this feature flag.

h/templates/client/settings/notifications.html

-<div ng-if="feature('notification')" class="tab-pane" title="Notifications">
+<div class="tab-pane" title="Notifications">
   <form class="account-form form" name="notificationsForm">
     <p class="form-description">Receive notification emails when:</p>
     <div class="form-field form-checkbox-list">