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').
