Only expose message_size_stats command to rabbitmq-diagnostics

The whole point of introducing rabbitmq-diagnostics,
rabbitmq-queues, rabbitmq-streams is to stop
growing the ctl command list, which has been massive
even five years ago.

(cherry picked from commit 42e99dd)

Author: michaelklishin

deps/rabbitmq_cli/lib/rabbitmq/cli/diagnostics/commands/message_size_stats_command.ex

@@ -11,7 +11,7 @@ defmodule RabbitMQ.CLI.Diagnostics.Commands.MessageSizeStatsCommand do
 
   @default_timeout 60_000
 
-  def scopes(), do: [:ctl, :diagnostics]
+  def scopes(), do: [:diagnostics]
 
   def switches(), do: [timeout: :integer]
   def aliases(), do: [t: :timeout]