chg: remove usage statistics (aka datachoices)

[mvrueden]

I created a discussion in the OpenNMS Discourse (here) with the goal to get a feeling about what can be removed from the OpenNMS Horizon source code.

I created a working branch, called develop-headless to be the basis for the next upcoming removals of the features mentioned in said above Discourse Discussion.

In this current Pull Request I am removing the usage statistics plugin.