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