stop double-logging error traces (#369)

manticore/core/executor.py

@@ -510,8 +510,6 @@ class Executor(object):
                     import traceback
                     trace = traceback.format_exc()
                     logger.error("Exception: %s\n%s", str(e), trace)
-                    for trace_line in trace.splitlines():
-                        logger.error(trace_line) 
                     #Notify this worker is done
                     self.will_terminate_state(current_state, current_state_id, 'Exception')
                     current_state = None