Cleanup: Remove unused uu_pname code

Coverity caught a possible NULL pointer dereference in dead code. We can
delete it all.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Chunwei Chen <david.chen@nutanix.com>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #13900