remove type & url attr of ulink in translation

Closes #395

Example of generated file:

help/pluma.pot before:

msgid "To report a bug or make a suggestion regarding the <application>pluma</application> application or this manual, follow the directions in the <ulink url=\"help:mate-user-guide/feedback\" type=\"help\">MATE Feedback Page</ulink>."
msgstr ""

help/pluma.pot after:

msgid "MATE Feedback Page"
msgstr ""

msgid "To report a bug or make a suggestion regarding the <application>pluma</application> application or this manual, follow the directions in the <_:ulink-1/>."
msgstr ""

help/C/index.docbook

@@ -16,8 +16,12 @@
 -->
 <!-- =============Document Header ============================= -->
 <?db.chunk.max_depth 2?>
-<article id="index" lang="en">
+<article id="index" lang="en" xmlns:its="http://www.w3.org/2005/11/its" its:version="2.0">
 <!-- please do not change the id; for translations, change lang to -->
+         <its:rules version="2.0">
+           <its:translateRule translate="no" selector="//ulink/@type|//ulink/@url"/>
+           <its:withinTextRule withinText="no" selector="//ulink"/>
+         </its:rules>
 <!-- appropriate code -->
   <articleinfo>
 	 <title>Pluma Manual<!-- not using app entity because of lowercase ugliness --></title>