Improve AWS single-tier documentation

Reorganize how-to-set-up-a-single-tier-system.adoc to mirror the actual
EC2 launch wizard flow, and fix several discrepancies between the doc
and the real deployment:

- Remove the architecture diagram and nginx/443/8443 references, which
  no longer match the deployment (Kaui listens directly on 3000, Kill
  Bill on 8000, no reverse proxy).
- Drop the hardcoded AMI version requirement, which goes stale as new
  versions ship.
- Explain what a public subnet is (route table with a 0.0.0.0/0 route
  to an Internet Gateway) and how to create one.
- Fix the security group instructions: correct ports are 3000/8000, not
  443/8443, and the SSH/22 rule is now added during network settings
  instead of as a separate post-launch step.
- Add a Storage step and explicit Name/tags, AMI, and Instance type
  guidance, matching the sections actually shown in the EC2 console.
- Update screenshots to match the current AWS Marketplace/EC2 UI.

Also remove tutorial-single-tier.adoc, an orphaned draft that duplicated
this guide with the same stale content and wasn't linked from anywhere.

Fixes killbill/technical-support#285

Author: sbrossie