Remove Feature Flag ci_prevent_file_var_expansion_downstream_pipeline

What does this MR do and why?

• Original Feature issue: #390252 (closed)
• Original MR where the feature flag was introduced: !124320 (merged)
• Rollout issue for Feature Flag: #414583 (closed)
• Steps followed for Feature Flag removal: #414583 (closed)
• Release Note: https://docs.gitlab.com/ee/update/deprecations.html#file-type-variable-expansion-fixed-in-downstream-pipelines!

The feature flag was enabled in production on May 6, 10:14 AM PST, and has been monitored since then.

Screenshot_2024-05-07_at_12.19.59_PM

Remove Feature Flag in downstream pipeline

When defining a new YAML variable in downstream pipeline (example below) that expands a file variable, the behaviour would change from expanding the content of the file, into expanding the file path to the file.

Changelog: changed

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