PCI/MSI: Remove unused mask_msi_irq() and unmask_msi_irq()

Change pcie-xilinx-nwl.c to use pci_msi_mask_irq() and pci_msi_unmask_irq()
like all other PCI host controller drivers.  Remove the now-unused
mask_msi_irq() and unmask_msi_irq().

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: Michal Simek <michal.simek@xilinx.com>
CC: linux-arm-kernel@lists.infradead.org