sim: Stop using loadState in the Root SimObject.

sim: Stop using loadState in the Root SimObject.

The primary difference between using loadState and letting the default
implementation of loadState call unserialize is that whether or not that code
is called is dependent on that object being associated with a section in the
checkpoint file being unserialized. Since there's always a "root" object,
there should always be a section for it in the checkpoint and those should be
equivalent.

This removes one custom implementation of the loadState function.

Change-Id: Ia674ccc18e141f38746e22ccfddc21475b1a0731
Reviewed-on: https://gem5-review.googlesource.com/4740
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>