selftests: mptcp: netlink: drop duplicate var ret

selftests: mptcp: netlink: drop duplicate var ret

The variable 'ret' are defined twice in pm_netlink.sh. This patch drops
this duplicate one that has been defined from the beginning, with
commit eedbc685321b ("selftests: add PM netlink functional tests")

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240223-upstream-net-next-20240223-misc-improvements-v1-5-b6c8a10396bd@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>