scons: Delete the now unused --update-ref option.

scons: Delete the now unused --update-ref option.

This option was for use with the old scons based regression tests.
Those had been deleted for a while, but some other bits and pieces
related to them were still lying around, depending on this option. Now
that those have been cleaned up, this option can go away.

Change-Id: I95b41dd0a14c51e74d1e527eb40e09f49aaeaf7e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32122
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>