Remove logback-nexus.xml template

The resource using this template was remove for 3.0.0. This is another instance of configuring a Nexus server after it has been installed / started - leave it alone and let a wrapper cookbook configure it. There are some additional concerns:

* the file is only written when the Nexus is started. Meaning we need to ensure the Nexus server is available before running the resource. Checking if the server is running requires a data bag (TODO: change this because it is bad)
* the format is different for Nexus 2.7 anyway.