Remove support for OAUTH_REDIRECT_AT_SIGN_IN (#17287)

Fixes #15959

Introduced in #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 #10232 from
appearing, and completely broke with the introduction of #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.

config/initializers/omniauth.rb

@@ -5,7 +5,6 @@ end
 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'