message: doc: remove mention of obsolete relayserver target
  this is the one from
  Cr-Commit-Position: refs/heads/master@{#30958}
diff: @@ -242,12 +242,6 @@ your connection. Open one more tab using the same page. Connect it too (with a
  different name). It is now possible to exchange messages between the connected
  peers.
  -### Relay Server
  -
  -Target name `relayserver`. Relays traffic when a direct peer-to-peer
  -connection can't be established. Can be used with the call application above.
  -
  -
  ### STUN Server
  Target name `stunserver`. Implements the STUN protocol for Session Traversal
  