Remove feature flag `issue_zoom_integration`

In !16609 (merged) we've added the feature flag issue_zoom_integration.

Let's remove it after we've completed testing.

• Remove feature flag flag from the code base ➡ !17787 (merged)
• Remove from staging via /chatops run feature delete issue_zoom_integration --staging
• Remove from production via /chatops run feature delete issue_zoom_integration

Related to #31103 (closed)