kobject: Remove redundant checks for whether ktype is NULL

When adding koject or kset, we have made sure that ktype cannot be NULL.
Therefore, after adding koject or kset, there is no need to worry about
ktype being NULL. Clear all ktype-related redundancy checks.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Link: https://lore.kernel.org/r/20230805084114.1298-3-thunder.leizhen@huaweicloud.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>