platform2: drop libchrome-180609 dependency

libchrome-180609 is deprecated and all platform2 targets are now based
on libchrome-242728.

BUG=chromium:361748
TEST=Trybot runs on paladin, release and chromiumos-sdk builders.

Change-Id: If40db94857435dcf48eddf4f9fe592c49a8b5974
Reviewed-on: https://chromium-review.googlesource.com/195565
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>