chore(CMakeLists): remove unused HAVE_UNISTD check
chore(CMakeLists): remove unused HAVE_UNISTD check

digama0/lean,johoelzl/lean,digama0/lean,fpvandoorn/lean,fpvandoorn/lean,rlewis1988/lean,digama0/lean,fpvandoorn/lean,rlewis1988/lean,avigad/lean,fgdorais/lean,Kha/lean,leanprover/lean,leanprover-community/lean,avigad/lean,johoelzl/lean,avigad/lean,avigad/lean,johoelzl/lean,rlewis1988/lean,johoelzl/lean,Kha/lean,johoelzl/lean,dselsam/lean,dselsam/lean,fgdorais/lean,leanprover/lean,leodemoura/lean,dselsam/lean,fgdorais/lean,Kha/lean,fpvandoorn/lean,leanprover-community/lean,Kha/lean,rlewis1988/lean,avigad/lean,fpvandoorn/lean,leanprover/lean,leanprover-community/lean,digama0/lean,leodemoura/lean,fgdorais/lean,leodemoura/lean,rlewis1988/lean,johoelzl/lean,dselsam/lean,leanprover/lean,leodemoura/lean,Kha/lean,digama0/lean,leanprover/lean,leanprover-community/lean,leodemoura/lean,leanprover/lean,leanprover-community/lean,rlewis1988/lean,avigad/lean,fpvandoorn/lean,fgdorais/lean,dselsam/lean,digama0/lean,leanprover-community/lean,dselsam/lean,Kha/lean,leodemoura/lean,fgdorais/lean