Remove GraphQL `feature_flag` property

What does this MR do and why?

This MR removes the GraphQL feature_flag property. The property had been renamed to _deprecated_feature_flag in an earlier deprecation step !93748 (merged).

This change removes it altogether and all code that was supporting it.

There should be no functional changes.

Issue: #369202 (closed)

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.

Related to #369202 (closed)