stop generation of Go client from this repository

I believe the Go client generated in the client dir is generated here out of convenience. This causes some inter-depenecy issues between this project and the gapic-generator-go. To help break this cycle we should stop generating the client and checking it in here and instead just generate the client on the fly in gapic-generator-go. This is the same pattern any other language would use to integrate with showcase.

Start by inspecting the generated Go client in the client directory and the repository configuration or scripts that produce and check it in. Trace how gapic-generator-go generates Showcase clients and identify references that assume the checked-in client exists. Done means this repository no longer generates or checks in the client and the gapic-generator-go flow can generate it on the fly.

Written by the indexing model from the issue text.