Revert "testing: add TB.SetGOMAXPROCS function"

Revert "testing: add TB.SetGOMAXPROCS function"

This reverts CL 519235.

Reason for revert: Proposal is still in incoming.

For #62020

Change-Id: Icccb930209f36097f5d930c01eda6b5042bdddc1
Reviewed-on: https://go-review.googlesource.com/c/go/+/573516
Reviewed-by: Than McIntosh <thanm@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>