refactor: remove unused webhook handler for wallets

it was only used in lnpay, opennode and alby which all had it disabled anayways with a pretty old TODO, so i guess the feature is disabled for a y ear now and nobody used it, so i just removed it

lnbits/app.py

@@ -61,7 +61,6 @@ from .tasks import (
     check_pending_payments,
     internal_invoice_listener,
     invoice_listener,
-    webhook_handler,
 )
 
 
@@ -465,10 +464,6 @@ def get_db_vendor_name():
 
 
 def register_async_tasks(app):
-    @app.route("/wallet/webhook")
-    async def webhook_listener():
-        return await webhook_handler()
-
     @app.on_event("startup")
     async def listeners():
         create_permanent_task(check_pending_payments)