doc: consolidate and update README.md, INSTALL.md (googleapis/google-cloud-cpp-spanner#1026)
doc: consolidate and update README.md, INSTALL.md (googleapis/google-cloud-cpp-spanner#1026)

* chore: remove subdir README.md
* doc: remove broken install links for this release

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