Unused (and undocumented) `kind` parameter in `read_excel`
closes #4563

Why does read_excel take a `kind` parameter at all? It gets passed to the `ExcelFile` constructor, set on the object and then (as far as I can tell) never gets used for anything.

This should either be removed from the signature (which doesn't affect anything, because it will be accepted into kwds anyways) or added into the docs and then actually used for something.

if we remove it, just need to add a:

``` python
# Remove this branch in 0.14
if 'kind' in kwargs:
   warnings.warn("Kind is deprecated and will be removed in a future version of pandas")
   kwargs.pop('kind')
```

Was not previously being used for anything anyways.
