isdn/eicon: don't call flush_scheduled_work() from diva_os_remove_soft_isr()

diva doesn't use workqueue and there is no reason to flush the system
workqueue from diva_os_remove_soft_isr().  Remove it.

This is to prepare for the deprecation and removal of
flush_scheduled_work().

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Armin Schindler <armin@melware.de>