Remove unused source attribute from translation files (#1370)

build/sortxtb.xslt

 <xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
-  <xsl:output method="xml" indent="yes" omit-xml-declaration="no" encoding="UTF-8" />
+  <xsl:output method="xml" indent="yes" omit-xml-declaration="no" encoding="UTF-8"/>
   <xsl:strip-space elements="*"/>
   <xsl:template match="translationbundle">
-      <!-- XSLT doesn't support adding doctype without PUBLIC or SYSTEM annotation, so add it to the doc raw -->
-      <xsl:text disable-output-escaping="yes">&lt;!DOCTYPE translationbundle&gt;
-</xsl:text>
+    <!-- XSLT doesn't support adding doctype without PUBLIC or SYSTEM annotation, so add it to the doc raw -->
+    <xsl:text disable-output-escaping="yes">&lt;!DOCTYPE translationbundle&gt;</xsl:text>
     <xsl:copy>
       <xsl:apply-templates select="@*"/>
       <xsl:apply-templates select="translation">
@@ -15,9 +14,13 @@
     </xsl:copy>
   </xsl:template>
 
+  <!--empty template suppresses 'source' attribute -->
+  <xsl:template match="@source"/>
+
+  <!--identity template copies everything forward by default-->
   <xsl:template match="@* | node()">
     <xsl:copy>
-      <xsl:apply-templates select="@* | node()" />
+      <xsl:apply-templates select="@* | node()"/>
     </xsl:copy>
   </xsl:template>
 </xsl:stylesheet>