Remove postgres cluster addon

[squizzi]

This PR deprecates the cluster addon postgres and removes any references to it in command Example's; effectively reverting #378.

Testing

cluster addon create postgres is no longer a command and produces help output when called, only object-store is left as available commands for cluster addon.

./bin/replicated cluster addon create postgres
Create new add-ons for a cluster. This command allows you to add functionality or services to a cluster by provisioning the required add-ons.


Usage:
  replicated cluster addon create [command]

Example:
  # Create an object store bucket add-on for a cluster
  replicated cluster addon create object-store CLUSTER_ID --bucket-prefix mybucket

  # Perform a dry run for creating an object store add-on
  replicated cluster addon create object-store CLUSTER_ID --bucket-prefix mybucket --dry-run

Available Commands:
  object-store Create an object store bucket for a cluster. <---

Flags:
  -h, --help   help for create

Global Flags:
      --app string                The app slug or app id to use in all calls
      --integration-test string   Set to the name of the integration test to run
      --log-api-calls string      Log the API calls to the specified file
      --token string              The API token to use to access your app in the Vendor API

Use "replicated cluster addon create [command] --help" for more information about a command.