Resolve "Ready UGA OpenAthens Go Live"

Related to #349 (closed)

• - homepage_controller.rb
  • no feature flag needed--just need to remove code after golive
• - sessions_controller.rb
  • no feature flag needed--just need to remove code after golive
• - welcome_controller.rb
  • no feature flag needed--just need to remove code after golive
• - application_helper.rb
  • no feature flag needed--just need to remove code after golive
• - open_athens_strategy.rb
  • feature flag controls auth type :cas vs. :oa