e1000: remove unused Kconfig option for disabling packet split

Since the e1000/e1000e split, no hardware supported by e1000
supports packet split, just remove the Kconfig option and associated
code from the driver.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>