some minor cleanup on include/ktap_types.h

remove some unused structure fields.

Signed-off-by: Jovi Zhangwei <jovi.zhangwei@gmail.com>
remove redundant macro ttisnil

Use isnil instead.

Signed-off-by: Jovi Zhangwei <jovi.zhangwei@gmail.com>
minor cleanup ktap_assert macro

Signed-off-by: Jovi Zhangwei <jovi.zhangwei@gmail.com>