Chore: Remove suggestion to install "google/cloud" package from all READMEs

[bshaffer]

In the README of our packages, including the root README (which maybe it's ok there) we have the following:

We also provide the google/cloud package, which includes all Google Cloud clients.

$ composer require google/cloud

We should remove this language from existing packages, and also from future packages in the README template. We do not want to recommend installing the entire package, and only recommend using the individual ones.