Revert "Use List-Id for threading"

This reverts commit bbc550ef1d.

app/src/main/java/eu/faircode/email/MessageHelper.java

@@ -785,11 +785,6 @@ public class MessageHelper {
 
     String[] getReferences() throws MessagingException {
         List<String> result = new ArrayList<>();
-
-        String listId = imessage.getHeader("List-Id", null);
-        if (!TextUtils.isEmpty(listId))
-            result.add(listId);
-
         String refs = imessage.getHeader("References", null);
         if (refs != null)
             result.addAll(Arrays.asList(getReferences(refs)));