Remove message box when editing topic of change

I do not recall why I added this message box below the topic name
field in ChangeScreen2.  I suspect the original idea was to permit
more notes to be added, for example why the topic is being assigned.

At the recent Gerrit hackathon it sounded like nobody ever uses this
box.  Remove it to simplify the UI.

Also remove message from the PutTopic REST API.  We suspect nobody
will notice the missing field.

Change-Id: I8d32287ee3b08d9409dd7d58688cd9effdfd380f

gerrit-server/src/main/java/com/google/gerrit/server/change/PutTopic.java

@@ -48,7 +48,6 @@ class PutTopic implements RestModifyView<ChangeResource, Input>,
   static class Input {
     @DefaultInput
     String topic;
-    String message;
   }
 
   @Inject
@@ -92,12 +91,7 @@ class PutTopic implements RestModifyView<ChangeResource, Input>,
           new ChangeMessage.Key(change.getId(), ChangeUtil.messageUUID(db)),
           currentUser.getAccountId(), TimeUtil.nowTs(),
           change.currentPatchSetId());
-      StringBuilder msgBuf = new StringBuilder(summary);
-      if (!Strings.isNullOrEmpty(input.message)) {
-        msgBuf.append("\n\n");
-        msgBuf.append(input.message);
-      }
-      cmsg.setMessage(msgBuf.toString());
+      cmsg.setMessage(summary);
 
       db.changes().beginTransaction(change.getId());
       try {