cbuildbot: do not print detailed stack trace

This is a partial revert of CL:241482 and also eliminates a caller to

TEST=Unit tests; git-grepping to ensure all non-unittest callers of
FormatDetailedTraceback are removed.

Change-Id: I968af6154ad88ac58ef43f119f1a47aa03c9a8a2
Reviewed-on: https://chromium-review.googlesource.com/262207
Reviewed-by: David James <davidjames@chromium.org>
Commit-Queue: Aviv Keshet <akeshet@chromium.org>
Tested-by: Aviv Keshet <akeshet@chromium.org>
4 files changed