[dataimport] remove dead code
The only caller of ObjectStore._put is ObjectStore.create_entity, which
RQLObjectStore overrides (and doesn't call up).
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/