app: Stop showing short description on installation page

Since there are currently no apps with short description, it does not show
already anyway.

Tests:

- Installing an app works.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: Joseph Nuthalapati <njoseph@riseup.net>

plinth/templates/app-header.html

@@ -27,9 +27,6 @@
           {% else %}
             <h2>{{ app_info.name }}</h2>
           {% endif %}
-          {% if app_info.short_description %}
-            <h3>{{ app_info.short_description }}</h3>
-          {% endif %}
         </div>
       {% endblock %}
       {% if app_enable_disable_form %}