Dedupe yarn.lock (#3452)

Summary:
I noticed that #3451 introduced duplicate versions of `webpack` by not updating some nested webpack dependencies.

This diff just deletes the yarn.lock to recreate it from scratch which seems to do a better job deduping.

Pull Request resolved: #3452

Reviewed By: alunyov

Differential Revision: D27679953

Pulled By: kassens

fbshipit-source-id: 3a9e6286f1b0c5152f4b14e9f504012b1403d696