lium: Remove lium from dev-util and update README.md

lium is now on GitHub: https://github.com/google/lium
To avoid confusion, removing the source and explain that in README.md.

BUG=None
TEST=None

Change-Id: I93f363946b3dc79ce9bc8fcec827b980e3159248
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/4565573
Tested-by: Hikaru Nishida <hikalium@chromium.org>
Commit-Queue: Masami Hiramatsu <mhiramat@google.com>
Reviewed-by: Masami Hiramatsu <mhiramat@google.com>
Auto-Submit: Hikaru Nishida <hikalium@chromium.org>
35 files changed