Cleanup Watchman trigger list

Summary:
As we are running a watchman per working directory (build directory) we don't
mind to shutdown the connected watchman instance. The State won't be saved and
the trigger-list will be deleted.

Test Plan: run falcon ; grep watchman ; stop falcon ; grep watchman

Reviewers: adrienconrath

Reviewed By: adrienconrath

Subscribers: NicolasDP

Maniphest Tasks: T23

Differential Revision: http://91.121.199.76/D44