Fix migration 20101026184950 down method

[spaghetticode]

When the up method of migration 20101026184950 runs after the rollback,
the column unlock_token already exists as it's added by the down method.

There's no apparent reason to keep adding that column in the down method, so
this commit removes it.

[kennyadsl]

Is it normal that both up and down create the same column? Maybe there's an error there?

[spaghetticode]

I may well be... I'm not 100% sure about that. Since my main goal was to be able to complete Solidus migrations rollback without generating errors, I did the most conservative choice.

This migration is still present in the original form also in the spree version of the gem: https://github.com/spree/spree_auth_devise/blob/master/db/migrate/20101026184950_rename_columns_for_devise.rb

Now I tried to create a sample app without add_column :spree_users, :unlock_token, :string in the down method and both migrations and rollbacks are still fine. If you agree I can change the down instead of the up method.

[kennyadsl]

Yes I think it's better. I think that line is just a mistake since that same column is removed at line 25 of that migration.

[kennyadsl]

@spaghetticode can you please rebase? I removed the CircleCI config

[kennyadsl]

Thanks @spaghetticode !