Remove mentions of Yarn from Node.js wrapper

While I think it (or pnpm) is still necessary to generate a project template, I don't think it's needed now that's done, I'd rather just use npm since it's bundled with Node.js, and having the two lockfiles is causing confusion.

nodejs/package.json

@@ -29,6 +29,5 @@
     "test": "ava",
     "universalize": "napi universalize",
     "version": "napi version"
-  },
-  "packageManager": "yarn@4.9.1"
+  }
 }