net: wwan: iosm: remove unused enum definition

ipc_time_unit enum 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/8295a6138f13c686590ee4021384ee992f717408.1683649868.git.m.chetan.kumar@linux.intel.com


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