Resolve "Improve Variables validation message"

What does this MR do?

Removes duplicate validation from Project/Group variables.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #43275 (closed)