teuthology/run_tasks: do not call sys.exc_clear() for clearing exception
teuthology/run_tasks: do not call sys.exc_clear() for clearing exception

sys.exc_clear() was removed in Python3, see
https://docs.python.org/3/whatsnew/3.0.html#index-22. so we should not
call it.

Signed-off-by: Kefu Chai <a4b264ee037c625052534d60505bfd0db4776ac4@redhat.com>

dmick/teuthology,SUSE/teuthology,dmick/teuthology,SUSE/teuthology,ktdreyer/teuthology,ceph/teuthology,ktdreyer/teuthology,SUSE/teuthology,dmick/teuthology,ceph/teuthology