cleanup: remove clutter from some samples (#1231)
cleanup: remove clutter from some samples (#1231)

When possible use single-line `if(..) throw..` to handler errors, and
avoid introducing `StatusOr` unless we really use it.
googleapis/google-cloud-cpp-spanner,googleapis/google-cloud-cpp-spanner,googleapis/google-cloud-cpp-spanner,googleapis/google-cloud-cpp-spanner