Revert of Another attempt: gclient: delete mismatching checkouts (https://codereview.chromium.org/183283003/)

Reason for revert:
Broke `gclient sync` for me, failing with:

Error: 1> Can't update/checkout /Volumes/MacintoshHD2/src/chrome-git/src if an unversioned directory is present. Delete the directory and try again.


For someone else, it broke it with:

% gclient sync

________ unmanaged solution; skipping src

Error: Command svn info --xml returned non-zero exit status 1 in /Users/pawliger/chromium/src/.

<?xml version="1.0" encoding="UTF-8"?>

<info>

svn: E155007: '/Users/pawliger/chromium/src' is not a working copy

Original issue's description:
> Another attempt: gclient: delete mismatching checkouts
> 
> Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=255930

TBR=iannucci@chromium.org,szager@chromium.org,maruel@chromium.org,mmoss@chromium.org,borenet@google.com
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/192323006

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@256005 0039d316-1c4b-4281-b951-d872f2087c98