[autotest] Use |json| instead of |simplejson|.

simplejson was moved into the library as of python 2.4. We're now at
python 2.6/2.7, and having to keep installing simplejson everywhere has
slowly turned into a hassle.  Since we can just trivially drop/rename
it, let's do so.

The rpc_interface parts of this change have been vetted, the rest is
done just as a s/simplejson/json/ across the code, which afaik should be
a safe thing to do.

BUG=None
TEST=unit, run_suite
DEPLOY=apache

Change-Id: Iab35f71157e7e5cd3be5291c776b07eb3283be8e
Reviewed-on: https://gerrit.chromium.org/gerrit/46424
Tested-by: Aviv Keshet <akeshet@chromium.org>
Reviewed-by: Alex Miller <milleral@chromium.org>
Commit-Queue: Aviv Keshet <akeshet@chromium.org>
diff --git a/frontend/tko/graphing_utils.py b/frontend/tko/graphing_utils.py
index 44a5e11..17a6ceb 100644
--- a/frontend/tko/graphing_utils.py
+++ b/frontend/tko/graphing_utils.py
@@ -22,7 +22,7 @@
 import StringIO, colorsys, PIL.Image, PIL.ImageChops
 from autotest_lib.frontend.afe import readonly_connection
 from autotest_lib.frontend.afe.model_logic import ValidationError
-from simplejson import encoder
+from json import encoder
 from autotest_lib.client.common_lib import global_config
 from autotest_lib.frontend.tko import models, tko_rpc_utils