PCI/MSI: Remove unused kobject from struct msi_desc

After commit 1c51b50c ("PCI/MSI: Export MSI mode using attributes, not
kobjects"), the kobject in struct msi_desc is unused.

Remove the unused struct kobject from struct msi_desc.

[bhelgaas: changelog]
Fixes: 1c51b50c ("PCI/MSI: Export MSI mode using attributes, not kobjects")
Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>