selftests: fix an unused variable warning in pidfd selftest

I fixed a few warnings like this in commit e2aa5e65
("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 <axelrasmussen@google.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>