container: remove unimplemented CreateCluster method
container: remove unimplemented CreateCluster method

Fixes #1026.

Change-Id: Idaa674468250ea349b8aa98b6946183af3ae90eb
Reviewed-on: https://code-review.googlesource.com/28730
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go