Remove support for unused features:
- timeout
- nag_timer and nag_max
- Popen start property
- callback functions for stdout and stderr

Bug: 984182
Change-Id: Ib2327119508a89d1e60f4a42c64b78d050f48092
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1745406Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>