`DROP CONSTRAINT` on Partitions errors out after `DETACH PARTITION`

```sql
SELECT citus_add_node('localhost',9700,groupid=>0);
CREATE TABLE table_to_fkey (a INT UNIQUE);
CREATE TABLE citus_local_partitioned (a INT UNIQUE REFERENCES table_to_fkey(a)) PARTITION BY RANGE(a);
CREATE TABLE citus_local_partition (a INT UNIQUE);
SELECT citus_add_local_table_to_metadata('citus_local_partitioned', true);
ALTER TABLE citus_local_partitioned ATTACH PARTITION citus_local_partition DEFAULT;
ALTER TABLE citus_local_partitioned DETACH PARTITION citus_local_partition;
ALTER TABLE citus_local_partition DROP CONSTRAINT citus_local_partitioned_a_fkey;
ERROR: constraint "citus_local_partitioned_a_fkey_102010" of relation "citus_local_partition_102010" does not exist
```