removing port setting

We don't need to do this via the buildpack, it can be set like any other jvm arg using the Gradle run script convention of using <code>&lt;APP_NAME&gt;_OPTS</code>

So, the following manifest file will work for my app called example-books and all the mapping between jvm args and env vars are consistently done in the same place.
```
---
applications:
- name: ratpack
  memory: 512M
  instances: 1
  host: rh-ratpack
  domain: cfapps.io
  path: ./build/distributions/example-books.zip
  buildpack: https://github.com/rhart/java-buildpack-with-ratpack.git
  env:
    EXAMPLE_BOOKS_OPTS: -Dratpack.port=$PORT
```