chore(internal/gensupport): remove Hook (#772)
chore(internal/gensupport): remove Hook (#772)

As this package is no longer external, this concept can not be
used by anyone. To accomplish a similar feat a person can always
provide there own middleware by passing in a custom http.Client.

Updates: #321
google/google-api-go-client,google/google-api-go-client,googleapis/google-api-go-client,googleapis/google-api-go-client