w1-gpio: remove erroneous __exit and __exit_p()

Commit 8a1861d9

 ("w1-gpio: Simplify & get rid of defines") changed
(apparently unknowingly) the driver to a hotpluggable platform-device
driver but did not not update the section markers for probe and remove
(to __devinit/exit, which have since been removed). A later commit fixed
the section mismatch for probe, but left remove marked with __exit.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Cc: stable <stable@vger.kernel.org> # 3.8
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>