Reduce customizations for generated client `gkehub`

[parthea]

There are a lot of customizations made to the generated for gkehub. See the customizations here https://github.com/googleapis/python-gke-hub/blob/6e9144cb5a9badd2f8a95cfaf2577f86b197ccf6/owlbot.py#L44

One issue is that there are sub-APIs configmanagement and multiclusteringress which are included under the versioned API google/cloud/gkehub/v1.

The generated import statements are

from google.cloud.gkehub.configmanagement.v1 import ...
from google.cloud.gkehub.multiclusteringress.v1 import ...

but they should be

from google.cloud.gkehub_v1 import configmanagement_v1
from google.cloud.gkehub_v1 import multiclusteringress_v1

because the sub-APIs configmanagement and multiclusteringress are under google/cloud/gkehub/v1. See here.