added helper

app/helpers/oauth_helper.rb

@@ -16,4 +16,8 @@ module OauthHelper
       [:twitter, :github, :google_oauth2].include?(name.to_sym)
     end
   end
+
+  def additional_providers
+    enabled_oauth_providers.reject{|provider| provider.to_s.starts_with?('ldap')}
+  end
 end

app/views/devise/sessions/_oauth_providers.html.haml

-- providers = enabled_oauth_providers.reject{|provider| provider.to_s.starts_with?('ldap')}
+- providers = additional_providers
 - if providers.present?
   .bs-callout.bs-callout-info{:'data-no-turbolink' => 'data-no-turbolink'}
     %span Sign in with:  

spec/helpers/oauth_helper_spec.rb

+require "spec_helper"
+
+describe OauthHelper do
+  describe "additional_providers" do
+    it 'returns appropriate values' do
+      [
+        [[:twitter, :github], [:twitter, :github]], 
+        [[:ldap_main], []],
+        [[:twitter, :ldap_main], [:twitter]],
+        [[], []],
+      ].each do |couple|
+        allow(helper).to receive(:enabled_oauth_providers) { couple.first }
+        additional_providers.should include(*couple.last)
+      end
+    end
+  end
+end
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