Remove integration_form_sections feature flag

What does this MR do and why?

The related feature has been released on production and the feature flag integration_form_sections enabled by default in YAML.

This MR cleans up the feature flag and removed some unused code around integration form sections.

Changelog: changed

Screenshots or screen recordings

See !82970 (merged)

How to set up and validate locally

Go to Project > Settings > Integrations > Jira. For example, http://127.0.0.1:3000/gitlab-org/gitlab-shell/-/integrations/jira/edit.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #352827 (closed)