chore(docs): delete obsolete code samples (#386)

* chore(docs): delete obsolete code samples

The "hello world" example for Go is here: https://github.com/GoogleCloudPlatform/golang-samples/tree/main/bigtable/helloworld and this code sample references several unmaintained repos and uses OpenCensus (we are encouraging OpenTelemetry these days).

* chore(docs): delete obsolete code samples

* chore(docs): delete obsolete code samples

* chore(docs): delete obsolete code samples

* chore(docs): delete obsolete code samples

* chore(docs): delete obsolete code samples

* chore(docs): delete obsolete code samples

* chore(docs): update README.md