e1000: Removing the unused macro PAGE_USE_COUNT()

Removing the unused macro PAGE_USE_COUNT(), since there is no more reference
to it. The last reference was removed by Jesse's commit number 
630b25cd.

Signed-off-by: Breno Leitao <leitao@linux.vnet.ibm.com>
Acked-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>