DEV: Remove old TODOs for message-id formats (#27196)

Introduced back in 2022 in
e3d495850d,
our new more specific message-id format for inbound and
outbound emails has now been in use for a very long time,
we can remove the support for the old formats:

`topic/:topic_id/:post_id.:random@:host`
`topic/:topic_id@:host`
`topic/:topic_id.:random@:host`

spec/lib/imap/sync_spec.rb

@@ -142,7 +142,7 @@ RSpec.describe Imap::Sync do
     end
 
     context "when the message id matches the receiver post id regex" do
-      let(:message_id) { "topic/999/324@test.localhost" }
+      let(:message_id) { "discourse/post/324@test.localhost" }
       it "does not duplicate incoming email" do
         incoming_email = Fabricate(:incoming_email, message_id: message_id)