Remove SPF type SPF record #1394

As mentioned in #1394 - In accordance with RFC 7208, offer only TXT RRs for SPF.
Agree with @Nebukadneza - but not sure how to go about telling people to remove the old record...

core/admin/mailu/ui/templates/domain/details.html

@@ -34,8 +34,7 @@
 <tr>
   <th>{% trans %}DNS SPF entries{% endtrans %}</th>
   <td><pre>
-{{ domain.name }}. 600 IN TXT "v=spf1 mx a:{{ hostname }} -all"
+{{ domain.name }}. 600 IN TXT "v=spf1 mx a:{{ hostname }} -all"</pre></td>
-{{ domain.name }}. 600 IN SPF "v=spf1 mx a:{{ hostname }} -all"</pre></td>
 </tr>
 {% if domain.dkim_publickey %}
 <tr>