build: remove unused rollup.config.js files (#28646)

Since we build and publish the individual packages
using Bazel and `build.sh` has been removed, we can
safely remove the `rollup.config.js` files which are no
longer needed because the `ng_package` bazel rule
automatically handles the rollup settings and globals.

PR Close #28646

.github/angular-robot.yml

@@ -59,7 +59,6 @@ merge:
       - "**/package.json"
       - "**/tsconfig-build.json"
       - "**/tsconfig.json"
-      - "**/rollup.config.js"
       - "**/BUILD.bazel"
       - "**/*.md"
       - "packages/**/integrationtest/**"