Note: This CL originally just removed a deprecated use of Remove use of DoGetTrySlaves, suggested in http://crrev.com/2442153002, then was expanded to remove DoGetTrySlaves, GetPreferredTrySlaves and GetTrySlavesExecuter since these are all deprecated and unused.

BUG=660453

Review-Url: https://codereview.chromium.org/2453823002