Remove packages_conan_allow_empty_username_channel feature flag

What does this MR do and why?

In !75106 (merged), we added support for blank user/channel in the Conan Packages Registry.

That change impacted the validations executed during the package upload. As such, we deployed this change incrementally using a feature flag.

The rollout issue is #346006 (closed).

The feature flag flag has been fully enabled on gitlab.com for 4 days without any hiccup.

As such, this MR removes the feature flag completely from the code base to properly release this change.

Screenshots or screen recordings

See !75106 (merged)

How to set up and validate locally

See !75106 (merged)

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.