Make notification unsubscription work

[Radiokot]

Purpose

Use the separate endpoint for unsubscription, so the token gets actually removed on the server.

Changes

• Use the unsubscrube endpoint to unsubscribe from all the notifications

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.