Bluetooth: ISO: Remove bt_iso_chan_get_type() API declaration

The API was never implemented.
The channel type can also be obtained using bt_iso_chan_get_info().
If we would have to implement this API, it would share a lot of the
implementation with bt_iso_chan_get_info().

Removing the API makes the ISO APIs more similar to the ACL APIs
where we don't have a special API to fetch the role.

Signed-off-by: Rubin Gerritsen <rubin.gerritsen@nordicsemi.no>