Cleanup old Rails.version check for db migration path

Author: carlosantoniodasilva

lib/generators/devise/orm_helpers.rb

@@ -25,11 +25,7 @@ def migration_exists?(table_name)
       end
 
       def migration_path
-        if Rails.version >= '5.0.3'
-          db_migrate_path
-        else
-          @migration_path ||= File.join("db", "migrate")
-        end
+        db_migrate_path
       end
 
       def model_path