Simplify dependency list

`openjdk-7-jre-headless` provides `java7-runtime-headless` and `java7-runtime`
already includes `java7-runtime-headless` (i. e. `openjdk-7-jre` depends on
`openjdk-7-jre-headless`).

* https://packages.debian.org/wheezy/java7-runtime-headless
* https://packages.debian.org/wheezy/openjdk-7-jre-headless
* https://packages.debian.org/wheezy/java7-runtime
* https://packages.debian.org/wheezy/openjdk-7-jre