python: Remove a call to reduce() from code_formatter.py.

python: Remove a call to reduce() from code_formatter.py.

The built in reduce method is no longer available in python 3. Besides
that, this particular bit of code is simpler and easier to read if
reduce is replaced with the also built in sum() method.

Change-Id: I6daca42494ea0534721dfcfb1f6058517cd482d9
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35941
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>