Remove adjacency checks

What does this MR do?

The adjacency checks were added as a sanity check to guard against concurrent modification. They have proved to be much more trouble than they are worth, since they don't work as expected in the presence of deleted designs.

Addresses: #300071 (closed)

This removes the adjacency checks completely, and relies on the client making sane requests.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.