selftest: af_unix: Silence -Wall warning for scm_pid.c.

selftest: af_unix: Silence -Wall warning for scm_pid.c.

-Wall found 2 unused variables in scm_pid.c:

scm_pidfd.c: In function ‘parse_cmsg’:
scm_pidfd.c:140:13: warning: unused variable ‘data’ [-Wunused-variable]
  140 |         int data = 0;
      |             ^~~~
scm_pidfd.c: In function ‘cmsg_check_dead’:
scm_pidfd.c:246:15: warning: unused variable ‘client_pid’ [-Wunused-variable]
  246 |         pid_t client_pid;
      |               ^~~~~~~~~~

Let's remove these variables.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250811215432.3379570-5-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>