Remove support for OAUTH_REDIRECT_AT_SIGN_IN

Fixes mastodon#15959

Introduced in mastodon#6540, OAUTH_REDIRECT_AT_SIGN_IN allowed skipping the log-in form
to instead redirect to the external OmniAuth login provider.

However, it did not prevent the log-in form on /about introduced by mastodon#10232 from
appearing, and completely broke with the introduction of mastodon#15228.

As I restoring that previous log-in flow without introducing a security
vulnerability may require extensive care and knowledge of how OmniAuth works,
this commit removes support for OAUTH_REDIRECT_AT_SIGN_IN instead for the time
being.

.env.nanobox

@@ -202,10 +202,6 @@ SMTP_FROM_ADDRESS=notifications@${APP_NAME}.nanoapp.io
 # Name of the pam service used for checking if an user can register (pam "account" section is evaluated) (nil (disabled) by default)
 # PAM_CONTROLLED_SERVICE=rpam
 
-# Global OAuth settings (optional) :
-# If you have only one strategy, you may want to enable this
-# OAUTH_REDIRECT_AT_SIGN_IN=true
-
 # Optional CAS authentication (cf. omniauth-cas) :
 # CAS_ENABLED=true
 # CAS_URL=https://sso.myserver.com/

app/controllers/auth/sessions_controller.rb

@@ -13,14 +13,6 @@ class Auth::SessionsController < Devise::SessionsController
   before_action :set_instance_presenter, only: [:new]
   before_action :set_body_classes
 
-  def new
-    Devise.omniauth_configs.each do |provider, config|
-      return redirect_to(omniauth_authorize_path(resource_name, provider)) if config.strategy.redirect_at_sign_in
-    end
-
-    super
-  end
-
   def create
     super do |resource|
       # We only need to call this if this hasn't already been
@@ -87,14 +79,6 @@ def after_sign_in_path_for(resource)
     end
   end
 
-  def after_sign_out_path_for(_resource_or_scope)
-    Devise.omniauth_configs.each_value do |config|
-      return root_path if config.strategy.redirect_at_sign_in
-    end
-
-    super
-  end
-
   def require_no_authentication
     super
 

config/initializers/omniauth.rb

@@ -5,7 +5,6 @@
 Devise.setup do |config|
   # Devise omniauth strategies
   options = {}
-  options[:redirect_at_sign_in] = ENV['OAUTH_REDIRECT_AT_SIGN_IN'] == 'true'
 
   # CAS strategy
   if ENV['CAS_ENABLED'] == 'true'