selftests: fix an unused variable warning in pidfd selftest

I fixed a few warnings like this in commit e2aa5e650b07
("selftests: fixup build warnings in pidfd / clone3 tests"), but I
missed this one by mistake. Since this variable is unused, remove it.

Signed-off-by: Axel Rasmussen <[e-mail]>
Reviewed-by: Christian Brauner <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/pidfd/pidfd_wait.c

···
 		.flags = CLONE_PIDFD | CLONE_PARENT_SETTID,
 		.exit_signal = SIGCHLD,
 	};
-	int ret;
 	pid_t pid;
 	siginfo_t info = {
 		.si_signo = 0,