Remove cloudbuild.yaml

[chanseokoh]

It looks like the file was introduced as a workaround because the legacy docker_push rule stopped working. (Correct me if I'm wrong.) With the new rules (PR: #280), it works again in my testing.

[chanseokoh]

But looks like there's an automatic publish hook using it (#283 (comment)), so I guess cloudbuild.yaml should stay for a while.

But now that #280 is merged, perhaps we could just have a single step at least.

steps:  
- name: gcr.io/cloud-builders/bazel
  args: ['run', '//:publish']

[chanseokoh]

A deviation that cloudbuild.yaml producing additional tags: #297

[donmccasland]

We are using cloudbuild and triggers to automatically build and publish images. We will not be changing that.