base-stats: Fixed System "work_item" stat name

The name of this stat was prefixed with 'system.'. Something which is
unecessary and undesirable for the stats output.

Change-Id: I873a77927e1ae6bb52f66e9c935e91ef43649dcd
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/41833
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>

src/sim/system.cc

@@ -483,7 +483,7 @@ System::regStats()
         std::stringstream namestr;
         ccprintf(namestr, "work_item_type%d", j);
         workItemStats[j]->init(20)
-                         .name(name() + "." + namestr.str())
+                         .name(namestr.str())
                          .desc("Run time stat for" + namestr.str())
                          .prereq(*workItemStats[j]);
     }