km/common: remove obsolete defines and header

These defines and the header keymile_hdlc_enet.h are obsolete
due to the removed hdlc code.

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Heiko Schocher <hs@denx.de>
cc: Wolfgang Denk <wd@denx.de>