Move all NeXML related modules and tests to a new Bio-NeXMLIO distrib…

…ution.

The NeXML related modules are scattered through the Bio namespace but
still compose a modular unit.  Remove them and their tests.  They are
now in a new bioperl repo named Bio-NeXMLIO to be released as new
distribution with the same name.

This also drops our dependency on Bio-Phylo.