sim: Deschedule existing events when destructing an event queue.

sim: Deschedule existing events when destructing an event queue.

Other objects in the simulation may try to deschedule their events when
destructed, and if they're cleaned up after the event queue is then
they might try to deschedule events on an event queue that no longer
exists.

Change-Id: I9452ce52fba78297ce3dc4b3884289b5e2f2574d
Reviewed-on: https://gem5-review.googlesource.com/c/14400
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>