Eliminate usage of the async npm module

[JustinBeckwith]

We almost never have a good reason to be using the async npm module, as node 8 includes native async/await support. We need to go through the following repos, and drop the dependency:

• googleapis/gce-images
• googleapis/nodejs-bigtable
• googleapis/nodejs-compute
• googleapis/nodejs-datastore
• googleapis/nodejs-proto-files
• googleapis/nodejs-spanner
• googleapis/nodejs-text-to-speech
• googleapis/nodejs-vision