Build: fix duplicate case of switch in BuildPlan.swift (#6389)

The way this `switch` is written currently triggers a warning:

```
Sources/Build/BuildPlan.swift:770:22: warning: case is already handled by previous patterns; consider removing it
                case .macro:
```

Since this `case .macro` on line 770 is never reached due to another `case .macro` covering it above on line 737, it seems safe to remove the unused case altogether.

Author: MaxDesiatov