Fix pg_dump --clean with partitioned indexes.

Fix pg_dump --clean with partitioned indexes.

We'd try to drop the partitions of a partitioned index separately,
which is disallowed by the backend, leading to an error during
restore.  While the error is harmless, it causes problems if you
try to use --single-transaction mode.

Fortunately, there seems no need to do a DROP at all, since the
partition will go away silently when we drop either the parent index
or the partition's table.  So just make the DROP conditional on not
being a partition.

Reported-by: jian he <[email protected]>
Author: jian he <[email protected]>
Reviewed-by: Pavel Stehule <[email protected]>
Reviewed-by: Tom Lane <[email protected]>
Discussion: https://postgr.es/m/CACJufxF0QSdkjFKF4di-JGWN6CSdQYEAhGPmQJJCdkSZtd=oLg@mail.gmail.com
Backpatch-through: 13