mac80211_hwsim: drop now unused work-queue from hwsim

The work-queue was used for deferred destruction of hwsim radios;
this does not work well with namespaces about to exit. The one
remaining user has been migrated, so drop the now unused work-queue
instance.

Signed-off-by: Martin Willi <martin@strongswan.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>