platform/x86: dell-wmi: remove sparse_keymap_free() calls

As sparse_keymap_setup() now uses a managed memory allocation for the
keymap copy it creates, the latter is freed automatically.  Remove all
calls to sparse_keymap_free().

As this reduces dell_wmi_input_destroy() to one line, replace all calls
to that function with direct calls to input_unregister_device().

Signed-off-by: Michał Kępień <kernel@kempniu.pl>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>