Remove the Devise Async gem.

The extra gem isn’t necessary anymore since Rails 4.2 has ActiveJob integration.

Resolves #15575.

CHANGELOG

@@ -5,6 +5,7 @@ v 8.8.0 (unreleased)
   - Fix error when visiting commit builds page before build was updated
   - Add 'l' shortcut to open Label dropdown on issuables and 'i' to create new issue on a project
   - Updated search UI
+  - Replace Devise Async with Devise ActiveJob integration. !3902 (Connor Shea)
 
 v 8.7.1 (unreleased)
   - Throttle the update of `project.last_activity_at` to 1 minute. !3848

Gemfile

@@ -19,7 +19,6 @@ gem "pg", '~> 0.18.2', group: :postgres
 
 # Authentication libraries
 gem 'devise',                 '~> 3.5.4'
-gem 'devise-async',           '~> 0.9.0'
 gem 'doorkeeper',             '~> 2.2.0'
 gem 'omniauth',               '~> 1.3.1'
 gem 'omniauth-auth0',         '~> 1.4.1'

Gemfile.lock

@@ -164,8 +164,6 @@ GEM
       responders
       thread_safe (~> 0.1)
       warden (~> 1.2.3)
-    devise-async (0.9.0)
-      devise (~> 3.2)
     devise-two-factor (2.0.1)
       activesupport
       attr_encrypted (~> 1.3.2)
@@ -918,7 +916,6 @@ DEPENDENCIES
   database_cleaner (~> 1.4.0)
   default_value_for (~> 3.0.0)
   devise (~> 3.5.4)
-  devise-async (~> 0.9.0)
   devise-two-factor (~> 2.0.0)
   diffy (~> 3.0.3)
   doorkeeper (~> 2.2.0)

app/models/user.rb

@@ -91,7 +91,7 @@ class User < ActiveRecord::Base
   devise :two_factor_backupable, otp_number_of_backup_codes: 10
   serialize :otp_backup_codes, JSON
 
-  devise :lockable, :async, :recoverable, :rememberable, :trackable,
+  devise :lockable, :recoverable, :rememberable, :trackable,
     :validatable, :omniauthable, :confirmable, :registerable
 
   attr_accessor :force_random_password

config/initializers/devise_async.rb

@@ -1 +0,0 @@
-Devise::Async.backend = :sidekiq