Relocate mod_cluster load balancing demo outside of the main repository

There is a couple of reasons the demo should be moved out of the main repository:

• The demo has been broken for a while now, which has not prevented us from releasing anyway.
• It doesn't have any mod_cluster dependencies.
• It's not production code.

Clearly, this doesn't belong in the main repo - it was there originally for convenience along the native module and docs which have been pulled out now too.