Attempt to remove JDKxx_BOOT_DIR from jenkins machine definitions

[sxa]

In most cases the scripts in the build-farm directory will autodetect the JDK location (Typically using the symlinks in /usr/lib/jvm or an equivalent elsewhere) and therefore does not require the "fixed" JDKxx_BOOT_DIR definitions on the jenkins agent definition. This means we should be able to get rid of those definitions (which are also typically fixed to a specific version and will therefore fail if it is upgraded, unlike the use of the symlink)

This should reduce the complexity of our environments.