Clean up defaulted child class destructors

[ninotarantino]

We have a lot of code like the following

class Parent {
public:
    virtual ~Parent() = default;
};

class Child  : public Parent {
public:
    ~Child() override = default;
};

Since the parent's destructor is already marked virtual, we don't need to provide any child destructors unless we actually need to do something in the destructor.