message: Remove move-ctor/assignment from `util::SecretValue`
  Make copy-assignment more efficient.
diff: @@ -118,21 +118,10 @@ class SecretValue {
  }
  SecretValue& operator=(const SecretValue& other) {
  - ptr_ = MakeSecretUniquePtr<T>(*other.ptr_);
  - return *this;
  - }
  -
  - SecretValue(SecretValue&& other) {
  - ptr_ = MakeSecretUniquePtr<T>();
  - ptr_.swap(other.ptr_);
  - }
  -
  - SecretValue& operator=(SecretValue&& other) {
  - ptr_.swap(other.ptr_);
  + *ptr_ = *other.ptr_;
  return *this;
  }
  - ~SecretValue() = default;
  T& value() { return *ptr_; }
  const T& value() const { return *ptr_; }
  