Fix crash when running rebalance command in a mixed cluster of 7.0 and 7.2 (redis#12604)

In redis#10536, we introduced the assert, some older versions of servers
(like 7.0) doesn't gossip shard_id, so we will not add the node to
cluster->shards, and node->shard_id is filled in randomly and may not
be found here.

It causes that if we add a 7.2 node to a 7.0 cluster and allocate slots
to the 7.2 node, the 7.2 node will crash when it hits this assert. Somehow
like redis#12538.

In this PR, we remove the assert and replace it with an unconditional removal.

Author: enjoy-binbin

src/cluster.c

@@ -4935,12 +4935,8 @@ int clusterDelSlot(int slot) {
     if (!n) return C_ERR;
 
     /* Cleanup the channels in master/replica as part of slot deletion. */
-    list *nodes_for_slot = clusterGetNodesInMyShard(n);
-    serverAssert(nodes_for_slot != NULL);
-    listNode *ln = listSearchKey(nodes_for_slot, myself);
-    if (ln != NULL) {
-        removeChannelsInSlot(slot);
-    }
+    removeChannelsInSlot(slot);
+    /* Clear the slot bit. */
     serverAssert(clusterNodeClearSlotBit(n,slot) == 1);
     server.cluster->slots[slot] = NULL;
     /* Make owner_not_claiming_slot flag consistent with slot ownership information. */