[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/lib/PY_VERSION/*-packages/cubicweb/server/
usr/lib/PY_VERSION/*-packages/cubicweb/hooks/
usr/lib/PY_VERSION/*-packages/cubicweb/sobjects/
usr/lib/PY_VERSION/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/