build(docs-infra): remove unnecessary workaround for RxJS in `ng-packages-installer` (#28510)

Since b43f8bc7d, RxJS does not need to be patched any more in the
top-level `node_modules/`, so we don't need to special-case RxJS in
`ng-package-installer` and use `node_modules/rxjs/`.

PR Close #28510

aio/tools/ng-packages-installer/index.js

@@ -166,11 +166,6 @@ class NgPackagesInstaller {
       }
     });
 
-    // FIXME: Temporarily use RxJS from root `node_modules/`.
-    if (peerDependencies.rxjs) {
-      peerDependencies.rxjs = `file:${ANGULAR_ROOT_DIR}/node_modules/rxjs`;
-    }
-
     return [mergedDependencies, peerDependencies];
   }