Remove ast from file before cloning

- Deletes the ast key from the file after it is used as a local var in
  the exports module
- Prevents performance regression when the file is cloned in vinyl.

Vinyl 0.3.0 introduced deep cloning of file properties. When contents of
ast are included, it takes a conderable amount of time to clone it.
These contents are used as a local variable and not needed.

This was detected when building a project with d3.js, which apparently
has a complex ast. Time drastically reduced from 1.5 minutes to 18s.

Vinyl Change:
https://github.com/wearefractal/vinyl/commit/e493187b3f2fd1485077f09e73e669407ac077d3#diff-168726dbe96b3ce427e7fedce31bb0bcR51