This reverts commit e8acd40a.

Reason for revert: broke windows builds :(

Original change's description:
> bot_update: pin depot_tools
> 
> gclient runhooks often calls scripts that depend on scripts in depot_tools.
> Currently bot_update only ensures that depot_tools is present in $PATH, but
> otherwise uses whatever depot_tools already specified in $PATH. We don't pin
> depot_tools before running a recipe, so using whatever depot_tools in $PATH
> is not hermetic.
> 
> Insert the path to the pinned depot_tools to the beginning of $PATH
> 
> R=​iannucci@chromium.org
> 
> Bug:
> 
> Change-Id: I31d04ba2ec7c409f89d58a51502ce3565e3a02a1
> Reviewed-on: https://chromium-review.googlesource.com/481072
> Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
> Commit-Queue: Nodir Turakulov <nodir@chromium.org>
> 

TBR=iannucci@chromium.org,nodir@chromium.org,phajdan.jr@chromium.org,chromium-reviews@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Change-Id: I1a70f0f54e9b9cea7fc2f9897e62f558204a4684
Reviewed-on: https://chromium-review.googlesource.com/481440Reviewed-by: Nodir Turakulov <nodir@chromium.org>
Commit-Queue: Nodir Turakulov <nodir@chromium.org>