platform/x86: ideapad-laptop: hide unused 'touchpad_store'

A readonly sysfs property must not have a 'store' function:

drivers/platform/x86/ideapad-laptop.c:438:16: error: 'touchpad_store' defined but not used [-Werror=unused-function]

We can either comment it out or remove the function entirely,
without a good reason one or or another I picked the second option.

Fixes: 7f363145 ("platform/x86: ideapad-laptop: Switch touchpad attribute to be RO")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>