clean up usage of the six module

Now that we only support Python 3, we can drop all six usage.

BUG=b:187789302
TEST=CQ passes

Change-Id: I92ec7022f1f92fccc7d82ef3f262557081c0fc1b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2910860
Tested-by: Mike Frysinger <vapier@chromium.org>
Auto-Submit: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Chris McDonald <cjmcdonald@chromium.org>
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
diff --git a/scripts/dep_tracker.py b/scripts/dep_tracker.py
index 9e0d1a2..9e276bc 100644
--- a/scripts/dep_tracker.py
+++ b/scripts/dep_tracker.py
@@ -22,8 +22,6 @@
 import os
 import stat
 
-from six.moves import map as imap
-
 from chromite.lib import commandline
 from chromite.lib import cros_logging as logging
 from chromite.lib import filetype
@@ -83,7 +81,7 @@
       self._pool = multiprocessing.Pool(jobs)
       self._imap = self._pool.map
     else:
-      self._imap = imap
+      self._imap = map
 
     self._files = {}
     self._ebuilds = {}