refactor: Apply suggestion from @copilot

-
  #245 (comment)

Author: gerardo-navarro

lib/omniauth/strategies/saml.rb

@@ -184,8 +184,6 @@ def valid_slo_relay_state?(relay_state)
       end
 
       def call_slo_relay_state_validator(validator, relay_state)
-        arity = validator.arity
-
         return validator.call if validator.arity.zero?
         return validator.call(relay_state) if validator.arity == 1
         validator.call(relay_state, request)