Remove read_organization_push_rules feature flag

What does this MR do and why?

Remove read_organization_push_rules feature flag

The read_organization_push_rules feature flag has been enabled by default and is no longer needed. This commit removes:

• Feature flag YAML definition
• Feature flag checks in application code
• Test contexts that stub the feature flag as disabled

The feature is now permanently enabled, using OrganizationPushRule by default when the update_organization_push_rules feature flag is enabled.

Changelog: changed

References

[Feature flag] Cleanup FF `read_organization_pu... (#577705 - closed)

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