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

closes #6894

Deprecate passing keyword ``copy`` to:
- ``DataFrame/Series/Panel/.xs``
- ``Panel.minor_xs``
- ``Panel.major_xs``

``.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').