base: Delete commented out versions of the format_integer function.

If they're needed, they'd be fairly easy to recreate and are also
available in the revision history.

Change-Id: I5cf5e4b1271ce488016464048de69bc643dee4d9
Reviewed-on: https://gem5-review.googlesource.com/7641
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>