[autotest] Delete redundant call to parent's __str__

BUG=chromium:682417
TEST=None

Change-Id: I2f01023ffaedb9ab572e6110849805399876bc39
Reviewed-on: https://chromium-review.googlesource.com/437611
Commit-Ready: Prathmesh Prabhu <pprabhu@chromium.org>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Richard Barnette <jrbarnette@google.com>
diff --git a/client/common_lib/error.py b/client/common_lib/error.py
index 4409789..b98897d 100644
--- a/client/common_lib/error.py
+++ b/client/common_lib/error.py
@@ -37,8 +37,6 @@
 
 class AutotestError(Exception):
     """The parent of all errors deliberatly thrown within the client code."""
-    def __str__(self):
-        return Exception.__str__(self)
 
 
 class JobError(AutotestError):