km/common: remove hdlc_enet implementation

The hdlc implementation for mgcoge was initially developed,
but later on not used. Remove the C files, the references
in mgcoge.c and the Makefile to decrease maintenance effort.

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