Revert "cmd/go: add workspace pruning mode"

This reverts CL 357169.

Reason for revert: appears to be failing on longtest SlowBot.¹

¹https://storage.googleapis.com/go-build-log/a97c855b/linux-amd64-longtest_7c9857d4.log

Change-Id: I3b94395671db78ed5fb2fb1019e7199e4ffbd272
Reviewed-on: https://go-review.googlesource.com/c/go/+/362249


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>