We no longer need to use the depot_tools in the system path.

Bug: angleproject:6102
Change-Id: I6772ceeb4c61e686d15b2638fb4348e93179e321
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2989671Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>