Cut DestructorGuard default ctor
Cut DestructorGuard default ctor

Summary:
[Folly] Cut `DestructorGuard` default ctor. It does not actually need one, but its presence can hurt.

```lang=c++
void callback() {
  // compiles, which can hurt:
  DestructorGuard(delayed);

  // intended:
  DestructorGuard guard(delayed);
}
```

Reviewed By: lbrandy

Differential Revision: D10413329

fbshipit-source-id: f84eb0de0290cb09f7ac358044c2ef2d71dd8cea

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly