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