fix(dreal/util/logging.cc): Turn off logging by default
fix(dreal/util/logging.cc): Turn off logging by default

So that external users won't see interal logging messages.

soonho-tri/dreal4,dreal/dreal4,dreal/dreal4,dreal/dreal4,soonho-tri/dreal4,dreal/dreal4,soonho-tri/dreal4,soonho-tri/dreal4