broker: remove unused tweetnacl-util dependency

Addresses review feedback — encoding utilities are implemented in
src/util/encoding.ts, tweetnacl-util was never imported.

Author: Baudbot

slack-broker/package-lock.json

@@ -11,8 +11,7 @@
     "typecheck": "tsc --noEmit"
   },
   "dependencies": {
-    "tweetnacl": "^1.0.3",
-    "tweetnacl-util": "^0.15.1"
+    "tweetnacl": "^1.0.3"
   },
   "devDependencies": {
     "@cloudflare/workers-types": "^4.20241230.0",