selftests: mptcp: pm: rm the right tmp file

"$err" is a variable pointing to a temp file. "$out" is not: only used
as a local variable in "check()" and representing the output of a
command line.

Fixes: eedbc685

 (selftests: add PM netlink functional tests)
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>