Remove concurrent ruby gem no longer needed

Gemfile

@@ -46,7 +46,6 @@ gem 'webpacker'
 gem 'whenever', require: false
 
 # Error Tracing
-gem 'concurrent-ruby'
 gem 'mnemosyne-ruby'
 gem 'newrelic_rpm'
 gem 'sentry-rails'

app/models/submission.rb

@@ -5,8 +5,6 @@ class Submission < ApplicationRecord
   include Creation
   include ActionCableHelper
 
-  require 'concurrent/future'
-
   CAUSES = %w[assess download file render run save submit test autosave requestComments remoteAssess
               remoteSubmit].freeze
   FILENAME_URL_PLACEHOLDER = '{filename}'

lib/docker_client.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require 'concurrent'
 require 'pathname'
 
 class DockerClient

lib/docker_container_pool.rb

@@ -1,8 +1,5 @@
 # frozen_string_literal: true
 
-require 'concurrent/future'
-require 'concurrent/timer_task'
-
 # get_container, destroy_container was moved to lib/runner/strategy/docker_container_pool.rb.
 # return_container is not used anymore because runners are not shared between users anymore.
 # create_container is done by the DockerContainerPool.