Remove redundant 'virtual' keywords from overridden methods

'virtual' is redundant when 'override' is present, so keep only
'override'.

Add t/t_dist_cppstyle.pl to check for this.

src/V3Slice.cpp

@@ -128,7 +128,7 @@ class SliceVisitor final : public VNVisitor {
         return newp;
     }
 
-    virtual void visit(AstNodeAssign* nodep) override {
+    void visit(AstNodeAssign* nodep) override {
         // Called recursively on newly created assignments
         if (!nodep->user1() && !VN_IS(nodep, AssignAlias)) {
             nodep->user1(true);
@@ -162,7 +162,7 @@ class SliceVisitor final : public VNVisitor {
         }
     }
 
-    virtual void visit(AstInitArray* nodep) override {
+    void visit(AstInitArray* nodep) override {
         UASSERT_OBJ(!m_assignp, nodep, "Array initialization should have been removed earlier");
     }
 
@@ -221,17 +221,17 @@ class SliceVisitor final : public VNVisitor {
             iterateChildren(nodep);
         }
     }
-    virtual void visit(AstEq* nodep) override { expandBiOp(nodep); }
-    virtual void visit(AstNeq* nodep) override { expandBiOp(nodep); }
-    virtual void visit(AstEqCase* nodep) override { expandBiOp(nodep); }
-    virtual void visit(AstNeqCase* nodep) override { expandBiOp(nodep); }
+    void visit(AstEq* nodep) override { expandBiOp(nodep); }
+    void visit(AstNeq* nodep) override { expandBiOp(nodep); }
+    void visit(AstEqCase* nodep) override { expandBiOp(nodep); }
+    void visit(AstNeqCase* nodep) override { expandBiOp(nodep); }
 
-    virtual void visit(AstNode* nodep) override { iterateChildren(nodep); }
+    void visit(AstNode* nodep) override { iterateChildren(nodep); }
 
 public:
     // CONSTRUCTORS
     explicit SliceVisitor(AstNetlist* nodep) { iterate(nodep); }
-    virtual ~SliceVisitor() override = default;
+    ~SliceVisitor() override = default;
 };
 
 //######################################################################