Fix the message threading for comment replies

We can't include the message writer's Account.Id in the Message-Id
as doing so prevents comments being sent by sendComment() from
threading under the earlier notifications sent by sendNewChange()
or sendNewPatchSet().

Signed-off-by: Shawn O. Pearce <sop@google.com>
diff --git a/src/main/java/com/google/gerrit/server/ChangeMail.java b/src/main/java/com/google/gerrit/server/ChangeMail.java
index 27f7a6b..4e2c6e9 100644
--- a/src/main/java/com/google/gerrit/server/ChangeMail.java
+++ b/src/main/java/com/google/gerrit/server/ChangeMail.java
@@ -529,10 +529,6 @@
     r.append(change.getCreatedOn().getTime());
     r.append('.');
     r.append(change.getChangeId());
-    if (fromId != null) {
-      r.append('.');
-      r.append(fromId.get());
-    }
     r.append('@');
     r.append(gerritHost());
     r.append('>');