cmd/go/testdata/script: remove reference to fixed issue

cmd/go/testdata/script: remove reference to fixed issue

The issue referenced in work_prune.txt was fixed in CL 362754, but we
forgot to update the TODO or mark the issue as fixed in the commit
message.

Updates #48331.

Change-Id: Id2115e496aeaef275dd3c8d32c5c5564e6addb07
Reviewed-on: https://go-review.googlesource.com/c/go/+/425485
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>