Remove Join Server Application Activation Settings

Summary

The Application Activation Settings in the Join Server are used as fallback for activation settings in the Join Server. These settings are only really relevant when working with external Join Servers. Since TTS Join Server is no longer intended to be used as a stand-alone Join Server (it's superseded by The Things Join Server in that role), these settings are no longer relevant.

References https://github.com/TheThingsNetwork/lorawan-stack/issues/3413

Current Situation

We currently have the AAS with rpcs to manage this: https://github.com/TheThingsNetwork/lorawan-stack/blob/v3.24/api/joinserver.proto#L328-L351 and below.

Why do we need this? Who uses it, and when?

We can remove this.

Proposed Implementation

Deprecate the API and remove it entirely. This is not used by the Console and there is no longer a CLI use case for this.

Contributing

[X] I can help by doing more research.
[X] I can help by implementing the feature after the proposal above is approved.
[X] I can help by testing the feature before it's released.

Code of Conduct

[X] I agree to follow TTN's Community Code of Conduct.

TheThingsNetwork / lorawan-stack