Disable Rails console's IRB's autocompletion by default

What does this MR do and why?

As mentioned in https://github.com/rails/rails/pull/46656, autocompletion increases data transmission significantly. This could cause noticeable slowdown when connecting to remote servers, which is usually the case in production.

How to set up and validate locally

1. In an Omnibus instance, copy irb.rb into /opt/gitlab/embedded/service/gitlab-rails/config/initializers.
2. Run gitlab-rails console.
3. Observe that typing Project. doesn't add autocompletion.