autotest: Do not log missing file for unsealed job directories

This is the expected path for all jobs that are not yet finished.
Logging (even at debug level) is creating too much span in gs_offloader
logs.

BUG=chromium:865561
TEST=None

Change-Id: I09eadaefc53282e874f23c5873177ddd063659bf
Reviewed-on: https://chromium-review.googlesource.com/1144180
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Allen Li <ayatane@chromium.org>
diff --git a/site_utils/job_directories.py b/site_utils/job_directories.py
index b1c04a9..e30a49b 100755
--- a/site_utils/job_directories.py
+++ b/site_utils/job_directories.py
@@ -259,8 +259,6 @@
       with open(marker_path) as f:
         ts_string = f.read().strip()
     except (OSError, IOError) as e:
-      logging.debug('Error opening %s for %s: %s',
-                    _OFFLOAD_MARKER, self.dirname, e)
       return None
     try:
       ts = int(ts_string)