Update Chat::Responder

Problem

While iterating over Identify users in Slack without a project or in... (&9418 - closed) epic and working on Remove association between ChatName and Integra... (#384974 - closed), we discovered a relation between chat_names and integration in the Chat::Responder module.

Having integration_id column removed from the chat_names table in !109197 (merged), the following code becomes obsolete:

def self.responder_for(build)
  integration = build.pipeline.chat_data&.chat_name&.integration

  if (responder = integration.try(:chat_responder))
    responder.new(build)
  end
end

The responder_for will always return a nil value, therefore we have to address this topic.

Proposal

1. REMOVE
   • Remove Chat::Responder logic, specs, and its usage (e.g. ChatNotificationWorker)
2. CHANGE
   • Update/adopt the logic (if applicable)

/cc @.luke @nicolasdular