BUG #6681: pg_extension.extconfig after alter sequence drop table

The following bug has been logged on the website:

Bug reference: 6681
Logged by: Philippe BEAUDOIN
Email address: phb(dot)emaj(at)free(dot)fr
PostgreSQL version: 9.1.4
Operating system: Linux
Description:

When a table has been registered as an extension configuration table, it's
oid is reported in the extconfig column of the pg_extension table. But once
the table is dropped from the extension, it is still referenced in
pg_extension.extconfig.
Here is a test case to easily reproduce this issue.
Just copy/paste and adjust initial parameters.

#!/bin/sh

export PGINST=/usr/local/pg913
export PGBIN=$PGINST/bin
export PGEXT=$PGINST/share/postgresql/extension

export PGHOST=localhost
export PGPORT=5913
export PGUSER=postgres
export PGDATABASE=phb

echo
"-----------------------------------------------------------------------"
echo " Issue: extconfig from pg_extension after alter sequence drop table"
echo
"-----------------------------------------------------------------------"
echo Connection parameters: $PGHOST - $PGPORT - $PGUSER - $PGDATABASE

# Build the extension control file
cat >$PGINST/share/postgresql/extension/phb.control <<EOF
default_version = '1.0.0'
schema = phb
EOF

# Create a test database
$PGBIN/psql template1 -ac "drop database phb;"
$PGBIN/psql template1 -ac "create database phb;"

# create the create extension script
cat >$PGEXT/phb--1.0.0.sql <<EOF
create table tbl1 (col1 int not null primary key, col2 int);
select pg_catalog.pg_extension_config_dump('tbl1','');
EOF
echo "--> Create the extension"
$PGBIN/psql -ac "create extension phb;"
$PGBIN/psql -ac "\dx+ phb"
$PGBIN/psql -ac "select oid from pg_class where relname = 'tbl1';"
$PGBIN/psql -ac "select * from pg_extension;"

echo "--> OK, tbl1 is correctly referenced as a configuration tables"
echo "--> Now remove tbl1 from the extension and look at the result"
$PGBIN/psql -ac "alter extension phb drop table phb.tbl1;"
$PGBIN/psql -ac "select * from pg_extension;"

echo "--> Surprisingly, extconfig references tbl1 !!!"
echo "--> However, tbl1 is really out of the extension."
$PGBIN/psql -ac "\dx+ phb"
echo "--> This is confirmed by a pg_dump output"
$PGBIN/pg_dump -Fp

echo "--> And the extension and the table can be dropped separately"
$PGBIN/psql -ac "drop extension phb;"
$PGBIN/psql -ac "\d phb.tbl1"
$PGBIN/psql -ac "drop table phb.tbl1;"

# cleanup the environment
$PGBIN/psql template1 -ac "drop database phb;"
rm $PGINST/share/postgresql/extension/phb*