Merge pull request #11279 from jetthoughts/removed_threadsafe_deprectaed_option

Removed deprecated threadsafe! from Rails Config

railties/lib/rails/application/configuration.rb

@@ -87,16 +87,6 @@ def paths
         end
       end
 
-      def threadsafe!
-        message = "config.threadsafe! is deprecated. Rails applications " \
-                  "behave by default as thread safe in production as long as config.cache_classes and " \
-                  "config.eager_load are set to true"
-        ActiveSupport::Deprecation.warn message
-        @cache_classes = true
-        @eager_load = true
-        self
-      end
-
       # Loads and returns the configuration of the database.
       def database_configuration
         yaml = paths["config/database"].first