feat: implement delete samples. (googleapis/google-cloud-cpp-spanner#396)
feat: implement delete samples. (googleapis/google-cloud-cpp-spanner#396)

Add samples that use DML to delete some data
(spanner_dml_standard_delete) an use mutations to delete data by key and
ranges (spanner_delete_data).

googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp