github(workflows/ubuntu.yml): Remove clang-9 in ubuntu focal
github(workflows/ubuntu.yml): Remove clang-9 in ubuntu focal

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