scons: Fix an exception in the DictImporter on scons shutdown.

scons: Fix an exception in the DictImporter on scons shutdown.

The DictImporter's __del__ method calls unload, and that imports
sys.modules so that it can remove the modules that the DictImporter had
set up as the importer goes away.

Unfortunately, the importer only goes away when python is shutting down,
and at that time some aspects of the system, namely sys.meta_path, have
been cleaned up. When unload tries to import sys, that causes an
exception which scons/python reports but which doesn't do anything bad
otherwise.

In all of the examples of this older style of import object online, none
had a __del__ method, and none worried about cleaning up sys.modules
when they went away. In light of that, I've removed the __del__ method
entirely.

Another reason I think it's safe to remove __del__ is that the importer
was not actually being deleted even when it was removed from
sys.meta_path, and all the modules it had loaded where removed from
sys.modules. I think that was because the SimObject classes that it had
set up still had references (they are used later in the SConscript), and
those would, either directly or indirectly, refer back to the modules
and the importer. Those remaining references kept the importer alive,
preventing __del__ from being called before all those other objects were
cleaned up.

I think in python 2, the order things were cleaned up just so happened
to avoid trying to import sys when it was no longer possible, but in
python 3 that changed and resulted in this exception being thrown.

I've tried building gem5 with scons running under python 2 and python 3,
and with this change there is no error at shutdown. Both also produce a
gem5 binary which can run hello world without problems.

Change-Id: Ib1f5c7403df57fc420cec7ec0fef20a164a06991
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27247
Tested-by: Gem5 Cloud Project GCB service account <345032938727@cloudbuild.gserviceaccount.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>