To remove MCB attributes, set the value of project.mcb.cpus to none and remove project.mcb.flags if set.
# projmod -s -K project.mcb.cpus=none new-project
# projects -l new-project new-project projid : 100 comment: "" users : (none) groups : (none) attribs: project.mcb.cpus=none project.pool=pool_default