apm821xx: nand remove obsolete kernel config symbol

apm821xx: nand remove obsolete kernel config symbol

The missing CONFIG_USB_XHCI_PCI dependency on CONFIG_USB_PCI of the
µPD720201 xhci-pci(e) usb 3.0 chip used by the WNDR4700 was fixed with
6112abf186bf ("kernel: enable CONFIG_USB_PCI for PCI usb modules").

Commit c38fb585763f ("kernel: fix disabling common usb quirks on 4.14")
finally fixed the following build error:

drivers/usb/host/xhci-pci.c: In function 'xhci_pci_probe':
drivers/usb/host/xhci-pci.c:749:6: error: implicit declaration of function 'usb_xhci_needs_pci_reset' [-Werror=implicit-function-declaration]
  if (usb_xhci_needs_pci_reset(dev)) {

Hence the CONFIG_USB_PCI symbol isn't required any longer in target kernel
config.

Signed-off-by: Mathias Kresin <dev@kresin.me>