ConcurrentHashTable::NoOp omits return in non-void return method

Static analysis complains there is a non-void return method without a return statement:

  struct NoOp {
    void operator()(VALUE*) {}
    const VALUE& operator()() {} // <--- here
    void operator()(bool, VALUE*) {}
  } noOp;

AFAICS, this is UB, and we have seen cases like these break compilers in other places. Not in this case, though, because noOp is only used as the default functor in remove. Still, it would be good to remove that risky definition, so that it is not used accidentally.