drivers: net: usb: pegasus: remove skb pool

The socket buffer pool for the receive path is now gone.  It's existence
didn't make much difference (performance-wise) and the code is better off
without the spinlocks protecting it.

Signed-off-by: Petko Manolov <petkan@nucleusys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>