Revert of Fetch Windows tools on Cygwin and use that Python for some scripts (https://codereview.chromium.org/140843004/)

Reason for revert:
This broke auto-updating depot_tools on Cygwin due to use of exec

Original issue's description:
> Fetch Windows tools on Cygwin and use that Python for some scripts
> 
> Tested by running gclient sync a couple of times from a Cygwin shell.
> 
> R=dpranke@chromium.org
> CC=scottmg@chromium.org,thakis@chromium.org,maruel@chromium.org
> BUG=335180
> 
> Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=245315

TBR=dpranke@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=335180

Review URL: https://codereview.chromium.org/132883007

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@245330 0039d316-1c4b-4281-b951-d872f2087c98