dashboard: shard openbsd-amd64-68 TryBot also
dashboard: shard openbsd-amd64-68 TryBot also

The goal in CL 279512 was to reduce numTryTestHelpers from 5 to 4,
but it unintentionally set the field only in the 386 builder, not
the amd64 one. This caused the amd64 OpenBSD TryBot to get very
slow (20-25 minutes), making it very much a bottleneck for overall
TryBot completion time.

Also stop explicitly setting some fields in openbsd-amd64-62
builder that have no effect by now, to simplify configuration.

For golang/go#17104.
Updates golang/go#35712.

Change-Id: I22823e4848cab65f11bde2a1cc70527929d0792d
Reviewed-on: https://go-review.googlesource.com/c/build/+/279723
Trust: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>

golang/build,golang/build,golang/build,golang/build