cue/build: remove unused Scope field

Change-Id: I4fd40af6e8ca92e215e77a84e66f027f86e32e92
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/9346
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Paul Jolly <paul@myitcv.org.uk>

cue/build/instance.go

@@ -49,11 +49,6 @@ type Instance struct {
 	loadFunc LoadFunc
 	done     bool
 
-	// Scope is another instance that may be used to resolve any unresolved
-	// reference of this instance. For instance, tool and test instances
-	// may refer to top-level fields in their package scope.
-	Scope *Instance
-
 	// PkgName is the name specified in the package clause.
 	PkgName string
 	hasName bool