pgsql: 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 <jian(dot)universality(at)gmail(dot)com>
Author: jian he <jian(dot)universality(at)gmail(dot)com>
Reviewed-by: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/CACJufxF0QSdkjFKF4di-JGWN6CSdQYEAhGPmQJJCdkSZtd=oLg@mail.gmail.com
Backpatch-through: 13

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/148cc0105dce93b52fe9e801f51ae0e3fbf96af1

Modified Files
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src/bin/pg_dump/pg_dump.c | 26 ++++++++++++++++++++------
1 file changed, 20 insertions(+), 6 deletions(-)