Allow all users to delete their account

What does this MR do?

Allow all users to delete their account

Previously the user could not delete his own account unless signup was enabled in the global settings. This was due to the fact that deleting the user would also delete the issues and other items associated to him. Deleting an user does not destroy essential data anymore, for this reason we removed the signup_enabled? check.

Are there points in the code the reviewer needs to double check?

No

Why was this MR needed?

For #36087 (closed)

Screenshots (if relevant)

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 #36087 (closed)