selftests/memory-hotplug: Remove the redundant warning information

Remove the redundant warning information of online_all_offline_memory()
since there is a warning in online_memory_expect_success().

Signed-off-by: Zhao Gongyi <zhaogongyi@huawei.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>