scons: Remove a workaround for a very old SCons bug.

I can't conclusively verify that this issue has been fixed because I
can't find any information about it on the internet. It's "SCons bug
2006" and "gem5 bug 308" but neither project still uses whatever bug
tracking mechanism those numbers correspond to. The change was mentioned
in the gem5 code base at least as early as 2008, and so hopefully has
been fixed in the last 13 years. I did a clean build without this
workaround, and things seem to work.

Change-Id: I155f0e72cb720fb5d35d8098375a45a69ea02263
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49419
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/SConscript

@@ -432,11 +432,6 @@ env.Append(CPPPATH=Dir('.'))
 for extra_dir in extras_dir_list:
     env.Append(CPPPATH=Dir(extra_dir))
 
-# Workaround for bug in SCons version > 0.97d20071212
-# Scons bug id: 2006 gem5 Bug id: 308
-for root, dirs, files in os.walk(base_dir, topdown=True):
-    Dir(root[len(base_dir) + 1:])
-
 ########################################################################
 #
 # Walk the tree and execute all SConscripts in subdirectories