Invariant check - Eliminate excess starter bricks

[fungairino]

• Start with a testing mod that has multiple excess starter brick definitions.
• Copy the mod, modify it and save it.
• Verify yaml structure does not have duplicates.

We need to be sure to test that the invariant checks are working correctly. That includes deleting excess starterBrick definitions. (See this PR for additional context: https://github.com/pixiebrix/pixiebrix-extension/pull/8557).