Remove editable preferences if stored in-memory (rather than in db)?

[brchristian]

@jhawthorn Now that in-memory preferences is default as of Solidus 2.4, I notice that preferences should no longer be editable from the admin backend. Instead, the new best practice is to include all preferences (e.g., FedEx credentials) in initializer files.

This creates a potentially problematic situation where the preferences displayed on the backend appear editable, but those changes will not persist after a deployment or a server reboot.

It seems to me that if a particular Solidus site is using in-memory preferences, we should either remove the ActiveShipping settings entirely, or make it read-only. Do you agree?

Happy to prepare a PR if so. Cheers!