Remove length limit that results failure in other tests.

We are not actually using this `limit` anywhere, but including this will fail tests such as [OptimisticLockingTest#test_removing_has_and_belongs_to_many_associations_upon_destroy](https://github.com/rails/rails/blob/master/activerecord/test/cases/locking_test.rb#L268) because the first name to insert is `Jonrun_before_createrun_before_validation`, which is unfortunately 41 characters long.

cc @senny
should be `null: false` though to stay consistent with https://github.com/rails/rails/blob/master/activerecord/test/schema/schema.rb#L528