Fix new / failing cluster slot migration test (#10482)

#10381 fixed an issue in `redis-cli --cluster reshard` that used to fail it (redis-cli) because
of a race condition.
the race condition is / was that when moving the last slot from a node, sometimes the PONG
messages delivering the configuration change arrive to that node before the SETSLOT arrives
to it, and it becomes a replica.
other times the the SETSLOT arrive first, and then PONG **doesn't** demote it.

**however**, the PR also added a new test that suffers from exactly the same race condition,
and the tests started failing a lot.

The fact is (if i understand it correctly), that this test (the one being deleted here), isn't related
to the fix that PR fixed (which was to fix redis-cli).
The race condition in the cluster code still happens, and as long as we don't solve it, there's
no reason to test it.

For now, even if my understandings are wrong, i'm gonna delete that failing test, since as far as
i understand, #10381 didn't introduce any new risks for that matter (which are gonna be
compromised by removing this check), this race existed since forever, and still exists, and the
fact that redis-cli is now immune to it is still being tested.

Additional work should be carried to fix it, and i live it for other PRs to handle.