Remove 'kerberos' from auth_helper.rb for gitlab-CE.

There is no Kerberos auth in gitlab-ce, so it shouldn't be noted
as a form-driven auth mechanism in app/helpers/auth_helper.rb.
This breaks using Kerberos as a custom omniauth provider.
See issue #2510

app/helpers/auth_helper.rb

 module AuthHelper
   PROVIDERS_WITH_ICONS = %w(twitter github gitlab bitbucket google_oauth2).freeze
-  FORM_BASED_PROVIDERS = [/\Aldap/, 'kerberos', 'crowd'].freeze
+  FORM_BASED_PROVIDERS = [/\Aldap/, 'crowd'].freeze
   def ldap_enabled?
     Gitlab.config.ldap.enabled