Remove Berksfile.lock from version control

A bug in Berkshelf 2.x makes this file pretty much useless: 
https://github.com/RiotGames/berkshelf/issues/637

The problem might be fixed in Berkshelf 3.x, but this version hasn't been
released yet. Therefore, remove Berksfile.lock to not raise expectations and
confuse people.

This reverts commit 8a28120396319381c64f165aeb86e5b5c4dff211.