Remove webpack-rails gem

• webpack-rails is no longer developed/supported
• GitLab barely uses this library anyway (pretty much just the logic for reading the manifest file)

We ought to take the one or two bits that we still use from webpack-rails and migrate them directly into GitLab's codebase along with all of our other webpack integration code.

This will make initiatives like gitlab-foss!18824 (closed) easier to implement in the future.