[vm] Remove dead VerifyCanonicalVisitor.

TEST=ci
Bug: #47378
Change-Id: Icd6debb309e2fd185330b089464f8934f1550190
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/431680
Reviewed-by: Alexander Aprelev <[email protected]>
Commit-Queue: Ryan Macnak <[email protected]>

Author: rmacnak-google

runtime/vm/heap/verifier.h

@@ -84,20 +84,6 @@ class VerifyWeakPointersVisitor : public HandleVisitor {
   DISALLOW_COPY_AND_ASSIGN(VerifyWeakPointersVisitor);
 };
 
-#if defined(DEBUG)
-class VerifyCanonicalVisitor : public ObjectVisitor {
- public:
-  explicit VerifyCanonicalVisitor(Thread* thread);
-  void VisitObject(ObjectPtr obj) override;
-
- private:
-  Thread* thread_;
-  Instance& instanceHandle_;
-
-  DISALLOW_COPY_AND_ASSIGN(VerifyCanonicalVisitor);
-};
-#endif  // defined(DEBUG)
-
 }  // namespace dart
 
 #endif  // RUNTIME_VM_HEAP_VERIFIER_H_