Revert "selftests: mptcp: simult flows: mark 'unbalanced' tests as flaky"

This reverts commit 052c9f0c.

If the test doesn't pass, we can get this error:

  # ./simult_flows.sh: line 275: mptcp_lib_subtest_is_flaky: command not found

This patch is not needed in v6.6: it is there to mark a test as "flaky",
but the MPTCP selftests infrastructure in v6.6 doesn't support them. So
it looks better to revert this patch.

Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>