Promote empty_repo_upload experiment

What does this MR do and why?

Promote the empty_repo_upload experiment and remove the experiment code

part of: #332046 #335721 (closed)

How to set up and validate locally

1. Create a new project without a readme: http://localhost:3000/projects/new#blank_project uncheck "Initialize repository with a README"

2. Verify there is an "Upload File" button, and that it works.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.