Assign thunk to null instead of using delete

[no-longer-on-githu-b]

For:

for (var i = 0; i < 1e6; ++i) {
  var x = defer(function() {
    return 1;
  });
  for (var j = 0; j < 100; ++j) {
    x.force();
    x.force();
    x.force();
    x.force();
  }
}

I get 16 seconds with delete and 9 seconds with null, consistently, in Chrome.

[garyb]

I thought I'd made this change a while back actually. delete should be avoided at all costs unless the thing is going to be checked with hasOwnProperty!