netlink: simplify socket destruction

Destroy the socket at the file descriptor close(2). There is no
reason to linger for any longer, there are no external references.
Remove pr_detach method as nothing left to do after pr_close.
Remove pr_abort method as it shall never be executed for this type
of socket.

Reviewed by: melifaro
Differential Revision: https://reviews.freebsd.org/D42521