remove secrets migration code

[jrhizor]

This was used for a one-time set of migrations. Doesn't seem worth maintaining after the migration, especially since it would require modifications with @lmossman's upcoming changes. We can always look at the old code and #7150 as a reference.

[lmossman]

LGTM

Wanted to confirm my understanding about this; Charles said that the idea with these one-time migrations is that we will require users on old enough versions to need to upgrade to a specific version before upgrading to the latest one, which will allow us to delete the code for these one-time migrations (e.g. this PR).

Does this PR mean that the "specific version" that they will need to upgrade to needs to be a version from before this PR is merged, so that they have the SecretsMigration class to use?

[lmossman]

As discussed in weekly planning, we should hold off on merging this until we are sure that we have everything we need for the intermediate migration

[jrhizor]

This is a script to introduce a specific feature that is only used (as far as we know) for Cloud. New instances configured with the secret store feature enabled would work.

Exporting from your old instance + importing your configs into the new instance is really how you should handle it. It's more complicated for cloud.

This is separate from Charles's discussion about one time breaking migrations for all users, so we should be safe to merge this.