PCI/MSI: Remove the conditional parent [un]mask logic

Now that msi_lib_init_dev_msi_info() overwrites the irq_[un]mask()
callbacks when the MSI_FLAG_PCI_MSI_MASK_PARENT flag is set by the parent
domain, the conditional [un]mask logic is obsolete.

Remove it.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Marc Zyngier <maz@kernel.org>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/all/20250903135433.444329373@linutronix.de