Remove unused IssuesChannel

This is no longer needed since we moved to GraphQL subscriptions for
this feature

app/channels/issues_channel.rb

-# frozen_string_literal: true
-
-class IssuesChannel < ApplicationCable::Channel
-  def subscribed
-    project = Project.find_by_full_path(params[:project_path])
-    return reject unless project
-
-    issue = project.issues.find_by_iid(params[:iid])
-    return reject unless issue && Ability.allowed?(current_user, :read_issue, issue)
-
-    stream_for issue
-  end
-end

app/services/issues/update_service.rb

@@ -43,10 +43,6 @@ module Issues
       ).execute(spam_params: spam_params)
     end
 
-    def after_update(issue)
-      IssuesChannel.broadcast_to(issue, event: 'updated') if Gitlab::ActionCable::Config.in_app? || Feature.enabled?(:broadcast_issue_updates, issue.project)
-    end
-
     def handle_changes(issue, options)
       old_associations = options.fetch(:old_associations, {})
       old_labels = old_associations.fetch(:labels, [])

spec/channels/issues_channel_spec.rb

-# frozen_string_literal: true
-
-require 'spec_helper'
-
-RSpec.describe IssuesChannel do
-  let_it_be(:issue) { create(:issue) }
-
-  it 'rejects when project path is invalid' do
-    subscribe(project_path: 'invalid_project_path', iid: issue.iid)
-
-    expect(subscription).to be_rejected
-  end
-
-  it 'rejects when iid is invalid' do
-    subscribe(project_path: issue.project.full_path, iid: non_existing_record_iid)
-
-    expect(subscription).to be_rejected
-  end
-
-  it 'rejects when the user does not have access' do
-    stub_action_cable_connection current_user: nil
-
-    subscribe(project_path: issue.project.full_path, iid: issue.iid)
-
-    expect(subscription).to be_rejected
-  end
-
-  it 'subscribes to a stream when the user has access' do
-    stub_action_cable_connection current_user: issue.author
-
-    subscribe(project_path: issue.project.full_path, iid: issue.iid)
-
-    expect(subscription).to be_confirmed
-    expect(subscription).to have_stream_for(issue)
-  end
-end

spec/services/issues/update_service_spec.rb

@@ -1024,10 +1024,8 @@ RSpec.describe Issues::UpdateService, :mailer do
           stub_feature_flags(broadcast_issue_updates: feature_flag_enabled)
 
           if should_broadcast
-            expect(IssuesChannel).to receive(:broadcast_to).with(issue, event: 'updated')
             expect(GraphqlTriggers).to receive(:issuable_assignees_updated).with(issue)
           else
-            expect(IssuesChannel).not_to receive(:broadcast_to)
             expect(GraphqlTriggers).not_to receive(:issuable_assignees_updated).with(issue)
           end