configs: Remove unneeded variable assignments in apu_se

configs: Remove unneeded variable assignments in apu_se

This patch removes:
A line assigning a variable to itself

An assignment to a variable (chroot) that is never used.
The above assignment also caused an error, "'NoneType' object
has no attribute 'startswith'"

Change-Id: Ib93c25fee4a0f7c1440de8067b086d8b96614796
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32654
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>