Stop using sum(1).

toybox doesn't have the BSD sum, and adding it doesn't seem like a good
idea in 2018. I assumed this was for Linux/Mac portability (because the
Mac doesn't have md5sum), but I'm informed that this isn't used on macOS
anyway.

Bug: N/A
Test: treehugger
Change-Id: I7b0d53fd14904778b5a1a01dfd8d9913407207ff