Fix possible incremental build issue

We tried to start using toybox's sed, but it produced invalid results
with one of init's genrules. Clear out that generated code to fix future
incremental builds, since we don't rebuild everything on a $PATH change.

Bug: 122744241
Test: generate bad file, remove toybox sed, apply this change
Change-Id: I197aa4b68b2fe309b3033e8736e1d80c8ded80ad