selftests: mptcp: removed defined but unused vars

Some variables were set but never used.

This was not causing any issues except adding some confusion and having
shellcheck complaining about them.

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