Revert "cmd/bench: make bent run with randomized builds"

This reverts CL 786660.

Reason for revert: changing the benchmarking mechanism confuses
comparison. Let's revert and rethink, and make the change at the
beginning of next cycle, so we get a clean comparison.

Change-Id: I9cb0c524414e2427222ec7071f2b4a22a98d2069
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/792220
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>