usb: plusb: remove unused pl_clear_QuickLink_features function

clang with W=1 reports
drivers/net/usb/plusb.c:65:1: error:
  unused function 'pl_clear_QuickLink_features' [-Werror,-Wunused-function]
pl_clear_QuickLink_features(struct usbnet *dev, int val)
^
This static function is not used, so remove it.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>