qom: Remove unused errp parameter from can_be_deleted()

qom: Remove unused errp parameter from can_be_deleted()

The errp argument is ignored by all implementations of the
method, and user_creatable_del() would break if any
implementation set an error (because it calls error_setg(errp) if
the function returns false).  Remove the unused parameter.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170829220337.23427-1-ehabkost@redhat.com>
Reviewed-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>