devlink: Remove unused extern declaration devlink_port_region_destroy()

devlink_port_region_destroy() is never implemented since
commit 544e7c33

 ("net: devlink: Add support for port regions").

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20230728132113.32888-1-yuehaibing@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>