tools: kwbimage: Remove unused enums and prototypes

There are more unused enums and function prototypes. Remove them. The
function kwbimage_check_params() does not return enum kwbimage_cmd_types,
but a boolean value returned as int.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>