removed unnecessary test case

This [test case](https://github.com/rails/rails/blob/master/activerecord/test/cases/associations/belongs_to_associations_test.rb#L622)  is sufficient to test invalid option for `belongs_to` `:dependent` options. So, [this](https://github.com/rails/rails/blob/master/activerecord/test/cases/associations/belongs_to_associations_test.rb#L628) is not required and if it is required then atleast we should use an existing valid `dependent` option other than`:delete, and :destroy` as `:restrict` is not used anymore in rails.