scons: Eliminate the "Arch" enum.

This is not the one from the object file loader, it's another one which
was only used by the System class. That use has been eliminated, so this
enum can be as well.

Change-Id: I476d7c1ef1bc1e34cbf904fc33c6735038e999c9
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48712
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu>

src/SConscript

@@ -692,17 +692,6 @@ def makeTheISA(source, target, env):
 #ifndef __CONFIG_THE_ISA_HH__
 #define __CONFIG_THE_ISA_HH__
 
-''')
-
-    # create an enum for any run-time determination of the ISA, we
-    # reuse the same name as the namespaces
-    code('enum class Arch {')
-    for isa in isas:
-        code('  $0,', namespace(isa))
-    code('};')
-
-    code('''
-
 #define IS_NULL_ISA ${{is_null_isa}}
 #define TheISA ${{namespace(target_isa)}}