platform/x86: thinkpad_acpi: Switch to using sparse-keymap helpers

Switch the hotkey keymap handling over to the sparse-keymap helpers,
there should be no functional changes from this.

Note all the mappings to KEY_UNKNOWN are removed since that is the default
behavior of sparse_keymap_report_event() for unknown scancodes.

Also drop the big comment about making changes to the keymaps since
the contents of that comment are mostly obsolete.
Tested-by: Mark Pearson <mpearson-lenovo@squebb.ca>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Mark Pearson <mpearson-lenovo@squebb.ca>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20240424122834.19801-20-hdegoede@redhat.com