Support drop partition for label rules

[rleungx]

Development task

We should support deleting the corresponding attributes from a partition immediately once the partition is dropped. e.g., we execute the following SQL:

create table t1 (c int) PARTITION BY RANGE (c)
    (PARTITION p0 VALUES LESS THAN (6),
     PARTITION p1 VALUES LESS THAN (11));
alter table t1 partition p0 attributes="attr";

We can get the corresponding attributes for p0 from PD.
Once we drop this partition,

alter table t1 drop partition p0;

We should not get any attributes of p0 from PD.