Remove crosh_builtin

This needs to stay over in chromiumos/assets, because that's where
build magic expects it to be.

BUG=None
TEST=None

Change-Id: I5ec75e1b9badaae475275f731391a1816a5f7116
Reviewed-on: https://chromium-review.googlesource.com/175887
Reviewed-by: Robert Ginda <rginda@chromium.org>
Tested-by: Robert Ginda <rginda@chromium.org>