master_remove_node crashes after removing reference table shard from metadata

```
brian=# CREATE TABLE ref (a int, b int);
CREATE TABLE
brian=# SELECT create_reference_table('ref');
create_reference_table
------------------------

(1 row)

brian=# DELETE FROM pg_dist_shard;
DELETE 1
brian=# SELECT master_remove_node('localhost', 9700);
server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.
```