T602 is an obsolete format, don't assume .txt files are T602

Without this, when the user tries to open a 0-bytes .txt file, they are
asked for a Save As operation after they hit the Save button.

When we remove the 'txt' from the T602 detection, it rather asks if the
user wants to use ODT or Plain text (and lose formatting).

Change-Id: Ic48fa61064a9ed78c64d56bc8864f0e12528e072
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/123216
Tested-by: Jenkins
Reviewed-by: Jan Holesovsky <kendy@collabora.com>

filter/source/config/fragments/types/writer_T602_Document.xcu

@@ -18,7 +18,7 @@
     <node oor:name="writer_T602_Document" oor:op="replace" >
         <prop oor:name="DetectService"><value>com.sun.star.comp.Writer.T602ImportFilter</value></prop>
         <prop oor:name="URLPattern"/>
-        <prop oor:name="Extensions"><value>602 txt</value></prop>
+        <prop oor:name="Extensions"><value>602</value></prop>
         <prop oor:name="MediaType"><value>application/x-t602</value></prop>
         <prop oor:name="Preferred"><value>true</value></prop>
         <prop oor:name="PreferredFilter"><value>T602Document</value></prop>