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/V3CoverageJoin.cpp

@@ -88,24 +88,24 @@ private:
     }
 
     // VISITORS
-    virtual void visit(AstNetlist* nodep) override {
+    void visit(AstNetlist* nodep) override {
         // Find all Coverage's
         iterateChildren(nodep);
         // Simplify
         detectDuplicates();
     }
-    virtual void visit(AstCoverToggle* nodep) override {
+    void visit(AstCoverToggle* nodep) override {
         m_toggleps.push_back(nodep);
         iterateChildren(nodep);
     }
     //--------------------
-    virtual void visit(AstNodeMath*) override {}  // Accelerate
-    virtual void visit(AstNode* nodep) override { iterateChildren(nodep); }
+    void visit(AstNodeMath*) override {}  // Accelerate
+    void visit(AstNode* nodep) override { iterateChildren(nodep); }
 
 public:
     // CONSTRUCTORS
     explicit CoverageJoinVisitor(AstNetlist* nodep) { iterate(nodep); }
-    virtual ~CoverageJoinVisitor() override {
+    ~CoverageJoinVisitor() override {
         V3Stats::addStat("Coverage, Toggle points joined", m_statToggleJoins);
     }
 };