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