Cleanup: Delete unnecessary pointer check from vdev_to_nvlist_iter()

This confused Clang's static analyzer, making it think there was a
possible NULL pointer dereference. There is no NULL pointer dereference.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14042