Remove confidential toggle checkbox while editing issue inline

What does this MR do?

Removes checkbox to toggle Issue Confidentiality while editing as same feature is available in Issue sidebar.

Screenshots

Before

After

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• [ ] Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • [ ] Has been reviewed by Backend
  • [ ] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #37454 (closed)