selftests: mptcp: join: remove unused vars

Shellcheck found that these variables were set but never used.

Note that rndh is no longer prefixed with '0-' but it doesn't change
anything.

Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>