net: wwan: iosm: remove unused macro definition

IOSM_IF_ID_PAYLOAD is defined but not used.
Remove it to avoid unexpected usage.

Signed-off-by: M Chetan Kumar <m.chetan.kumar@linux.intel.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/0697e811cb7f10b4fd8f99e66bda1329efdd3d1d.1683649868.git.m.chetan.kumar@linux.intel.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>