SECURITY: drop leaked alchemy_key from config.live.example.json

The key does not belong in any committed file — the worker reads the RPC
from ALCHEMY_RPC_URL env (or the gitignored config.json locally). Rotating
the key is the real fix (it hit the public repo history); this removes it
from HEAD.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

config.live.example.json

@@ -42,7 +42,6 @@
     "auto_redeem": true,
     "rpc_url": ""
   },
-  "alchemy_key": "yl8QpF2qEI_mQlhOdWgOi",
   "stake_cap_usd": 250.0,
   "wallets": [
     {