cmd/go: revert "output coverage report even if there are no test files"

cmd/go: revert "output coverage report even if there are no test files"

Original CL description:

    When using test -cover or -coverprofile the output for "no test files"
    is the same format as for "no tests to run".

Reverting because this CL changed cmd/go to build test binaries for
packages that have no tests, leading to extra work and confusion.

Updates #24570
Fixes #25789
Fixes #26157
Fixes #26242

Change-Id: Ibab1307d39dfaec0de9359d6d96706e3910c8efd
Reviewed-on: https://go-review.googlesource.com/122518
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>