Failed  to decommission BE

**Describe the bug**
This afternoon,  I wanted to decommission a BE in A cluster, but got the following error:
```
mysql> ALTER SYSTEM DECOMMISSION BACKEND "10.25.170.9:9050";
ERROR 1064 (HY000): Table[xx] in database[B:xxx] need more than 5 available backends to meet replication num requirement
```
The database[B:xxx] is belong to B cluster, not A cluster.
