Remove an unused variable.

    diag_uid.c:151:24:
    error: unused variable 'udr'
    [-Werror,-Wunused-variable]

Fixes: ac011361 ("af_unix: Add test for sock_diag and UDIAG_SHOW_UID.")
Signed-off-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20231124171645.1011043-4-willemdebruijn.kernel@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>