[autotest] Drop email notification from drone_utility

I haven't seen this notification in months. We don't want email
notifications from the lab.
Importing this module adds ~1 second to every single drone_utility call
from the scheduler. So this adds ~2 seconds to every tick in the lab.

BUG=chromium:718181
TEST=None

Change-Id: I6868271e71a06e1038ab008dbcc3e5f92d4fff61
Reviewed-on: https://chromium-review.googlesource.com/559937
Commit-Ready: Prathmesh Prabhu <pprabhu@chromium.org>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
diff --git a/scheduler/drone_utility.py b/scheduler/drone_utility.py
index dd4849d..e4d4b1a 100755
--- a/scheduler/drone_utility.py
+++ b/scheduler/drone_utility.py
@@ -36,7 +36,6 @@
 from autotest_lib.client.common_lib import utils
 from autotest_lib.client.common_lib.cros import retry
 from autotest_lib.scheduler import drone_logging_config
-from autotest_lib.scheduler import email_manager
 from autotest_lib.scheduler import scheduler_config
 from autotest_lib.server import subcommand
 
@@ -243,8 +242,7 @@
                 out_file.write("%s> %s\n" % (time.strftime("%X %x"), command))
                 out_file.write(separator)
             except (OSError, IOError):
-                email_manager.manager.log_stacktrace(
-                    'Error opening log file %s' % log_file)
+                pass
 
         if not out_file:
             out_file = open('/dev/null', 'w')