staging: delete STE RMI4 hackish driver

As of commit 62d5bdf9


"Merge branch 'synaptics-rmi4' into next" the input subsystem
has a proper RMI4 infrastructure and touchscreen driver.
The ST Ux500 platform has been converted to use the new driver
and its devicetree bindings. Delete this ancient hack.

Cc: Andrew Duggan <aduggan@synaptics.com>
Cc: Christopher Heiny <cheiny@synaptics.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>