drivers/net: Kconfig and Makefile cleanup

After the move of the Ethernet drivers into drivers/net/ethernet/
there was some leftover cleanup to do in the Kconfig and Makefile.

Removed the 10/100, 1000, and 10GbE Kconfig menus.

Removed the out-dated pci-skeleton.c file which was used an
example driver.  With the current networking features and
structure, the file is no longer a good example to use for
driver creation.

CC: Jeff Garzik <jgarzik@pobox.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>