infra: remove unused {TARGET, HOST}_PATH definition

Since the variables TARGET_PATH and HOST_PATH are not used anymore,
let's remove them.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>