Remove allow_concurrency setting

Gitlab::Runtime.multi_threaded? now requires access to Gitlab.config
which isn't available early on in these initializers.

We don't really need this setting anyway because this just wraps the app
in a mutex so that it can only be called by one thread at a time.

We're already running Puma by default so we're confident that our
application works without this mutex.

config/application.rb

@@ -22,7 +22,6 @@ module Gitlab
     require_dependency Rails.root.join('lib/gitlab/middleware/basic_health_check')
     require_dependency Rails.root.join('lib/gitlab/middleware/same_site_cookies')
     require_dependency Rails.root.join('lib/gitlab/middleware/handle_ip_spoof_attack_error')
-    require_dependency Rails.root.join('lib/gitlab/runtime')
 
     # Settings in config/environments/* take precedence over those specified here.
     # Application configuration should go into files in config/initializers

config/environments/development.rb

@@ -49,8 +49,6 @@ Rails.application.configure do
   # Do not log asset requests
   config.assets.quiet = true
 
-  config.allow_concurrency = Gitlab::Runtime.multi_threaded?
-
   # BetterErrors live shell (REPL) on every stack frame
   BetterErrors::Middleware.allow_ip!("127.0.0.1/0")
 

config/environments/production.rb

@@ -77,6 +77,4 @@ Rails.application.configure do
   config.action_mailer.raise_delivery_errors = true
 
   config.eager_load = true
-
-  config.allow_concurrency = Gitlab::Runtime.multi_threaded?
 end