Fully remove previous ID when performing `file.clone()`

This fixes https://github.com/docpad/docpad-plugin-multiplelayouts/issues/9 - I believe all 4 deletions are necessary due to the way things get munged together.
bah, semicolons!

I'm still thinking in JavaScript...
Merge pull request #893 from nfriedly/patch-1

Fully remove previous ID when performing `file.clone()`