mfd: wm831x-core: Drop unused module infrastructure from non-modular code

The Kconfig currently controlling compilation of this code is:

drivers/mfd/Kconfig:config MFD_WM831X
drivers/mfd/Kconfig:    bool

...meaning that it currently is not being built as a module by anyone.

Lets remove the couple traces of modular infrastructure use, so that
when reading the driver there is no doubt it is builtin-only.

We delete the MODULE_LICENSE tag etc. since all that information
is already contained at the top of the file in the comments.

Previous demodularizaion work has made wm831x_device_exit() no longer
used, so it is also removed from the 831x core code.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>