Roll back change list 2838024.

The current autotest ebuild cros_workon change takes more time than I expected, so I had to roll back this change to enforce autotest prebuild on buildbot.

As of now, the prebuild autotest tarball has no binaries produced within it.

Review URL: http://codereview.chromium.org/2881007

autotest

@@ -185,7 +185,6 @@ def build_autotest(options):
   environ['TEST_LIST'] = test_list
   environ['USE'] = use_flag
   emerge_cmd = ['emerge-%s' % options.board,
-                '-uDn',
                 'chromeos-base/autotest']
   if emerge_jobs:
     emerge_cmd.append(emerge_jobs)