Remove `require_organization_on_projects` feature flag

What does this MR do and why?

Removes the require_organization_on_projects as part of the rollout process #501373 (closed)

The feature flag has been enabled for at least 3 days https://gitlab.slack.com/archives/C101F3796/p1731039119982349 and has deemed to be stable. There has been no obvious service degradation nor any kibana/sentry logs that shows any breakage related to the flag.

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Related to #501373 (closed)