platform/x86: thinkpad_acpi: Use correct keycodes for volume and brightness keys

Change the default keymap to report the correct keycodes for the volume and
brightness keys. Reporting key events for these is already filtered out by
the hotkey_reserved_mask which masks these keys out of hotkey_user_mask at
initialization time, so there is no need to also map them to KEY_RESERVED.

This avoids users, who want these to be reported, having to also remap
the keycodes on top of overriding hotkey_user_mask to report these
and Linux userspace has already been overriding the KEY_RESERVED mappings
with the correct keycodes through udev/hwdb/60-keyboard.hwdb for years now.

Also drop hotkey_unmap() it was only used to dynamically map the brightness
keys to KEY_RESERVED and after removing that it has no remaining users.

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/20240424122834.19801-18-hdegoede@redhat.com