Clean mod cache should print remove commands and not run them when with set "-n" option.
Fixes #27458.

Change-Id: I97242cb40c062b347784cdb61653c84a3a7eab44
GitHub-Last-Rev: 5a6f10cad8c5f2c3916a74ca5eea27b1fdd1dc38
GitHub-Pull-Request: golang/go#27710
Reviewed-on: https://go-review.googlesource.com/135695
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>