platform/x86: intel-vbtn: Remove unused KEYMAP_LEN define

Now that we have 2 separate input_dev-s for the buttons and the switches,
this is no longer used, remove it.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20210325123255.73103-1-hdegoede@redhat.com