systemc: Remove a redundant injectException for Thread's throw_it.

systemc: Remove a redundant injectException for Thread's throw_it.

For some reason lost to the sands of time, the throw_it function was
virtual for the Thread class, and that class would call the base
class's throw_it, and then also injectException itself. That would
result in the exception being injected into the thread twice which is
incorrect.

Since it's not clear what the original intention of this code was, the
throw_it function is now no longer virtual, and the one useful aspect
of it, a check if the process is already terminated, was moved into the
base class function.

Change-Id: I7fb14baa7728bd1e9206011870b6ccaa9c4e8c64
Reviewed-on: https://gem5-review.googlesource.com/c/13312
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>