[Rails5] Remove `as` keyword from `Gitlab:Ci:Trace:Stream.delegate :valid?`

What does this MR do?

Rails 5.0 does not support the as keyword for delegate.

This MR removes this keyword from delegate :valid?

Are there points in the code the reviewer needs to double check?

I couldn't find where the present? is used by any instance of Gitlab::Ci::Trace::Stream.
Maybe I am missing something, but pipelines are green, so 🤷

Why was this MR needed?

Migration to Rails 5.0

Screenshots (if relevant)

No.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

#14286 (closed) and !12841 (closed)