net: ip: remove incorrect check of work item status

Checking whether a work item is pending before submitting it is racy:
the item may be finishing up in its handler, and essentially
completed, in which case chosing not to resubmit would leave work
unhandled.

In this case it appears very wrong, since the sole call site in
net_if.c has just initialized the work item, which is not permitted if
the work item is pending.

Remove the check.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>