sim: Don't serialize a root.isa string in checkpoints.

We are moving away from having a single ISA in a simulation. That means
it will no longer make sense to have a single, particular ISA at the
root of the object hierarchy which applies to the entire simulation.

Jira Issue: https://gem5.atlassian.net/browse/GEM5-1056

Change-Id: If4d354ac423e7ab4d3efbc6544d4feea93f56ab3
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48884
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/sim/root.cc

@@ -42,7 +42,6 @@
 #include "base/hostinfo.hh"
 #include "base/logging.hh"
 #include "base/trace.hh"
-#include "config/the_isa.hh"
 #include "debug/TimeSync.hh"
 #include "sim/core.hh"
 #include "sim/cur_tick.hh"
@@ -206,8 +205,6 @@ void
 Root::serialize(CheckpointOut &cp) const
 {
     SERIALIZE_SCALAR(FullSystem);
-    std::string isa = THE_ISA_STR;
-    SERIALIZE_SCALAR(isa);
 
     globals.serializeSection(cp, "globals");
 }