[forms] Do not raise errors on composite required relation being deleted The entity included in the composite will be deleted in such a case, so the field is not required even when the cardinality is '1' or '+'. Related to #8529868

usr/bin/cubicweb-ctl usr/bin/
usr/lib/PY_VERSION/*-packages/cubicweb/cwctl.py
../cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl