cmd/compile/internal/importer: remove // UNREVIEWED comments from tests

cmd/compile/internal/importer: remove // UNREVIEWED comments from tests

The importer tests are exact copies of the corresponding test files in
$GOROOT/src/go/internal/gcimporter/testdata/. With these changes, diff
applied recursively doesn't show any differences anymore among the
existing files between the directories

	src/cmd/compile/internal/importer/testdata
	src/go/internal/gcimporter/testdata

Change-Id: I1fdd70a3d4d5e55f1fc580892d1d38cabbdab81b
Reviewed-on: https://go-review.googlesource.com/c/go/+/351309
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>