Drop the default value on `deployments.status` column

Description

Currently, when you load rails application (e.g. via bundle exec rspec), you will see the following warning.

Both Deployment and its :status machine have defined a different default for "status". Use only one or the other for defining defaults to avoid unexpected behaviors.

According to https://github.com/pluginaweek/state_machine/issues/279, this is because the state_machine's default value and database's default value are different. In fact, the former is 0 (:created), and the latter is 2 (:success), today.

There was a reason why it ended up like so, however, after we have finished the add_column_with_default migration, we can drop the default value on status column as it should be handled by state machine (i.e. application level).

Proposal

Drop the default value on status column in 11.6