svn:externals are slow to update and are unnecessary in a gclient environment.

TEST=none
BUG=55904

Review URL: http://codereview.chromium.org/3421013

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@71604 0039d316-1c4b-4281-b951-d872f2087c98