sim: Eliminate the now unused System::getArch().

Change-Id: Ia36c9c3779719faba30105cd15f4495c8b6141c8
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48711
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu>

src/sim/system.hh

@@ -389,11 +389,6 @@ class System : public SimObject, public PCEventScope
      */
     AddrRangeList getShadowRomRanges() const { return ShadowRomRanges; }
 
-    /**
-     * Get the architecture.
-     */
-    Arch getArch() const { return Arch::TheISA; }
-
     /**
      * Get the guest byte order.
      */