stop generation of Go client from this repository

[codyoss]

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.