Remove "<!--" magic from XML files

It's not enough to recognise an XML file on its own.

Closes: #120

freedesktop.org.xml.in

@@ -6560,7 +6560,6 @@ command to generate the output files.
     <generic-icon name="text-html"/>
     <magic priority="40">
       <match type="string" value="&lt;?xml" offset="0"/>
-      <match type="string" value="&lt;!--" offset="0"/>
     </magic>
     <glob pattern="*.xml"/>
     <glob pattern="*.xbl"/>

tests/list

@@ -88,6 +88,8 @@ test.xht application/xhtml+xml
 test-html-with-svg.html text/html
 # Copied from https://gitlab.freedesktop.org/xdg/shared-mime-info/issues/98
 test-really-html-not-xhtml.html text/html oxx
+# Copied from https://gitlab.freedesktop.org/xdg/shared-mime-info/issues/120
+test.html text/html
 # Copied from http://cvs.fedoraproject.org/viewvc/comps/comps-f11.xml.in?revision=1.306
 test.xml.in application/xml x
 # Copied from http://bugs.freedesktop.org/show_bug.cgi?id=25581