pinctrl: remove orphaned exported ".remove" function

The Kconfig currently controlling compilation of this code is:

drivers/pinctrl/freescale/Kconfig:config PINCTRL_MXS
drivers/pinctrl/freescale/Kconfig:	bool

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

It also doesn't have any modular functionality, so it doesn't need
module.h included at all.

What it does have is an exported function that was used as a shared
".remove" by other drivers, but those use cases (imx23 and imx28)
are now gone, and hence this can disappear as well.

Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: linux-gpio@vger.kernel.org
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>