Remove the down peer once some stores cannot be back

## Enhancement Task
Consider that we have 4 stores and the replica is 3. If there are 2 stores broken, we might do the online recovery, which will tombstone those broken stores forcibly. After online recovery, there will be 2 voters and 2 learners for each region. But the learner which is in the broken stores will never come back. In this case, we can remove it directly since it won't be promoted to the voters, and also it won't block the `remove-tombstone` command.

Start by tracing the online recovery flow and the handling of learner stores after tombstoning, then inspect how the remove-tombstone command determines what blocks it. Done means learners in stores that cannot return are removed safely without being promoted to voters and no longer block remove-tombstone.

Written by the indexing model from the issue text.