Internals: Use C++11 = default where obvious. No functional change intended.

src/V3Slice.cpp

@@ -225,7 +225,7 @@ class SliceVisitor : public AstNVisitor {
 public:
     // CONSTRUCTORS
     explicit SliceVisitor(AstNetlist* nodep) { iterate(nodep); }
-    virtual ~SliceVisitor() override {}
+    virtual ~SliceVisitor() override = default;
 };
 
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