Remove feature flag integration_form_refactor

Remove the feature flag integration_form_refactor and all related code.

Previous MRs related to the feature flag (in chronological order):

Title	Link
Update Active checkbox component to use toggle	!27778 (merged)
Update Jira comment to include more information	!30258 (merged)
Extract integration trigger fields to Vue	!31074 (merged)
Extract integration custom fields to Vue	!31430 (merged)
Update ActiveToggle and JiraTriggerFields with new form layout	!32677 (merged)
Refine UI of integration form	!34707 (merged)