e1000e: Remove legacy jumbo frame receive code

The legacy jumbo frame receive code is no longer needed since all
hardware can do packet split and we're no longer offering a bypass
kernel config option to disable packet split. Remove the unused code.

Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>