net: lwm2m: remove unused multi_max_count field from obj_field

The multi_max_count is no longer used and can be removed from the
obj_field structure if we change all of the OBJ_FIELD() macros to
use OBJ_FIELD_DATA() instead.

Technically, OBJ_FIELD() and OBJ_FIELD_DATA() are now the same, but
we're keeping them both for the time being.  In the future, more
fields may be added to the obj_field structure and we can use the
OBJ_FIELD() macro again if that's the case.

Signed-off-by: Michael Scott <mike@foundries.io>