config: remove dead code in fs.py

config: remove dead code in fs.py

We have not added the --generate-dtb option for non-ARM systems and
hence this case becomes dead code. It also leads to error on non-ARM
systems as is tries to access a non existent field.

Change-Id: Ia926bd0c61efa275bc5e3864b8a9c3ffb7aa3cb5
Signed-off-by: Nayan Deshmukh <nayan26deshmukh@gmail.com>
Reviewed-on: https://gem5-review.googlesource.com/7801
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>