Remove unused wagtailimages_tags from group edit page

The `wagtailimages_tags` library was needlessly loaded on the group edit
page. This causes an error on installations that do not use
`wagtailimages`, or test suites for Wagtail plugins that do not use
images directly.

wagtail/wagtailusers/templates/wagtailusers/groups/edit.html

@@ -1,5 +1,5 @@
 {% extends "wagtailadmin/base.html" %}
-{% load wagtailusers_tags wagtailimages_tags static compress i18n %}
+{% load wagtailusers_tags static compress i18n %}
 
 {% block titletag %}{% trans "Editing" %} {{ group.name }}{% endblock %}