systemc: Fix a couple memory errors.

systemc: Fix a couple memory errors.

Because events are held in vectors, it doesn't make sense to keep an
iterator into the parent to keep track of where that event is for easy
removal since the iterator becomes invalid when the vector is changed.

The events need to be stored in a vector because systemc defines an
accessor which returns that vector, and building a vector on the fly
would be cumbersome.

Also, make sure the Event parent pointer is set to nullptr if there
isn't a parent.

Change-Id: I63a676190e7747e60baaca50009161d47bfc1c54
Reviewed-on: https://gem5-review.googlesource.com/12039
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>