Revert "PCI: Add x86_msi.msi_mask_irq() and msix_mask_irq()"

The problem fixed by 0e4ccb15 ("PCI: Add x86_msi.msi_mask_irq() and
msix_mask_irq()") has been fixed in a simpler way by a previous commit
("PCI/MSI: Add pci_msi_ignore_mask to prevent writes to MSI/MSI-X Mask
Bits").

The msi_mask_irq() and msix_mask_irq() x86_msi_ops added by 0e4ccb15
are no longer needed, so revert the commit.

default_msi_mask_irq() and default_msix_mask_irq() were added by
0e4ccb15

 and are still used by s390, so keep them for now.

[bhelgaas: changelog]
Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: David Vrabel <david.vrabel@citrix.com>
CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
CC: xen-devel@lists.xenproject.org