API: remove the copy kw from .xs to prevent an expectation of a view (which may not be possible)

closes #6894

You cannot try to pass ``copy=False`` to ``.xs`` (or the Panel variants)
as you may *think* that you are getting a view but you may not be

``.xs`` returns a view (as does ``.loc``) **IF POSSIBLE**

MultiIndexing with Slicers eliminates the need for this (and ``.xs`` for the too, but for
some cases its 'simpler').