platform2: drop libchrome-242728 dependency libchrome-242728 is deprecated and all platform2 targets are now based on libchrome-271506. BUG=chromium:375032 CQ-DEPEND=CL:201017 CQ-DEPEND=CL:200997 TEST=Trybot runs on paladin, release and chromiumos-sdk builders. Change-Id: I04b9b60d456c856b05d5e00039e81dc326205d13 Reviewed-on: https://chromium-review.googlesource.com/201013 Reviewed-by: Mike Frysinger <[email protected]> Tested-by: Ben Chan <[email protected]> Commit-Queue: Ben Chan <[email protected]>