util.h: Remove INLINE definition and user

INLINE is a very common macro, just like MAX or MIN.
Defining it always can easily collide with libraries or
application headers.
And option would be to add a ifdef guard around it,
But it was used in only 1 place in Zephyr, instead
of keeping it just for that, remove it.

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>