lib/file: remove sysfs_lowest_smbus

This function is now unused with Beltino platform gone.

BUG=chromium:990438
TEST=compiles

Change-Id: I05857162651fd4b34fe319528a9d84d964a1554f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/mosys/+/2392517
Tested-by: Jack Rosenthal <jrosenth@chromium.org>
Reviewed-by: Paul Fagerburg <pfagerburg@chromium.org>
Commit-Queue: Jack Rosenthal <jrosenth@chromium.org>
18 files changed