Input: cyttsp4 - remove driver

The cyttsp4 touchscreen driver was contributed in 2013 and since then
has seen no updates. The driver uses platform data (no device tree
support) and there are no users of it in the mainline kernel. There were
occasional fixes to it for issues either found by static code analysis
tools or via visual inspection, but otherwise the driver is completely
untested.

Remove the driver.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/ZrAZ2cUow_z838tp@google.com


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>