Remove a registered webhook URL

[ok300]

This is an umbrella-issue to track all necessary changes for this feature.

• Add support to remove webhook URL
  • LSPD (for payment notifications): Remove Webhook Subcription URL lspd#194
  • Breez server: Add Unsubscribe Notification Definitions to LSP RPC server#44
  • chainnotifier (for swap notifications): https://github.com/breez/chainnotifier/pull/6
  • SDK & CLI: Unregister webhook #920
• SDK: When changing LSP, remove webhooks from old LSP
• Add docs snippet: Unregister webhook breez-sdk-docs#151

Fixes #730

[JssDWt]

SDK: When changing LSP, remove webhooks from old LSP

Maybe this isn't necessary. Note that if you already have a channel with that LSP, it could be useful to keep the webhook available for if you're receiving a payment over that channel.

[dangeross]

Closing as all tasks are merged