README: Remove old, dev VM 'how to run the app' instructions
README: Remove old, dev VM 'how to run the app' instructions

- These instructions referenced the deprecated GOV.UK Development VM, `bowler`, \*and\* `./startup.sh`.
- The new standard is [GOV.UK Docker](https://github.com/alphagov/govuk-docker).

alphagov/asset-manager,alphagov/asset-manager,alphagov/asset-manager