doc: Remove tutorials/ovn-basics.

doc: Remove tutorials/ovn-basics.

The only thing worse than a lack of documentation is incorrect or
out-of-date documentation.  Over time, this document has not kept up with
the pace of OVN and is no longer a good current resource.

For a sandbox based tutorial like this, I'd like to start over using
ovn-trace as the basis.

An even more important type of tutorial would be something along the lines
of: http://blog.spinhirne.com/p/blog-series.html

That blog series was fantastic and has been the primary tutorial reference
I have been sending people to since it was written.

Signed-off-by: Russell Bryant <russell@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>