CLI: drop an unused import

(cherry picked from commit 5c7d643)

Author: michaelklishin

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

@@ -8,7 +8,6 @@ defmodule RabbitMQ.CLI.Diagnostics.Commands.CheckCertificateExpirationCommand do
   alias RabbitMQ.CLI.Core.DocGuide
   alias RabbitMQ.CLI.TimeUnit, as: TU
 
-  import RabbitMQ.CLI.Core.Platform, only: [line_separator: 0]
   import RabbitMQ.CLI.Core.Listeners
 
   @behaviour RabbitMQ.CLI.CommandBehaviour