use efficient print(e) instead of printf("%s", e.tostring)

We still can use e.tostring, but in most case, use print(e) will be
more efficient.

Also update exmaples in README to make use this change.
remove scripts/probe-by-id/*

Since ktap already support tracepoint/kprobe/uprobe now, so there
don't have any need to use that ugly demo script anymore.