WebODF viewer is now part of ViewerJS, so remove outdated README-viewer.odt

That odt talked about "The WebODF Viewer" and how to create and use it.

I have not created an updated version instead yet, because I am not sure what to tell exactly.

That the shell of ViewerJS is living in WebODF still puzzles me and how to explain that. Given that ViewerJS did not end up as dead project, but instead got a little bit of momentum, would it not be good to move the shell code over to ViewerJS and just keep the plugin code here?

In any case, this ODT here is outdated and wrong, thus should be removed.