serial: 8250_pci: Remove unused function get_pci_irq()

It is not used since commit 8428413b ("serial: 8250_pci: Implement MSI(-X) support")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: https://lore.kernel.org/r/20200909135103.17768-1-yuehaibing@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>