Remove inactive_projects_deletion feature flag

What does this MR do and why?

This feature flag was added with an intention to roll out inactive projects deletion feature on GitLab.com, however, since we're not doing that anymore, there is no need of this feature flag as well.

Screenshots or screen recordings

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

Edited by Huzaifa Iftikhar