Remove the Go custom-handler example (Go is now a first-class preview)

Now that Go is a first-class language for Azure Functions (currently in public preview), the Go example in this custom-handlers sample is no longer the recommended path for Go developers and should be removed to avoid steering people to a worse approach.

## Suggested scope
- Remove the Go example folder.
- Update the top-level README to note that Go is now supported natively (preview) and link to the canonical Go quickstart / docs.
- Keep the other language custom-handler examples in place.