Remove redundant cloud tests

We're currently generating a bunch of "real" clients to test against, that are probably redundant at this point (and are taking a lot of time during test runs and CI). We should remove most generated clients from the test cases, and generally keep only showcase and garbage.

In particular, we should remove from the cloud generator:
* language_v1beta1
* language_v1beta2
* servicemanager_v1beta1
* servicemanager_wrapper
* speech_v1
* vision_v1

And we should also remove from the default generator:
* language
* speech
* vision

I think we should retain language_v1 for cloud for now, so we keep the samples testing.
We should also generate wrappers for showcase, and maybe also garbage, for cloud.