Remove add_on_purchase_expires_on feature flag
What does this MR do and why?
Remove add_on_purchase_expires_on feature flag
- sufficiently enabled and verified in production #499907 (comment 2199213741)
- see #499907 (closed) and https://gitlab.com/gitlab-org/gitlab/-/issues/499720 for details
Changelog: other EE: true
MR acceptance checklist
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Related to #499907 (closed)