cmd/bent: remove outdated references to rsync and cp

Since CL 617555 rsync exec's were replaced with sweet's copyDir.
This removes misleading mentions of rsync and cp.

Change-Id: I78ffbac2683f80b6c6c3a17c653a01e50c831c89
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/674395
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>