Remove assertion for ALTER TABLE .. DETACH PARTITION CONCURRENTLY

One code path related to this flavor of ALTER TABLE was checking that
the relation to detach has to be a normal table or a partitioned table,
which would fail if using the command with a different relation kind.

Views, sequences and materialized views cannot be part of a partition
tree, so these would cause the command to fail anyway, but the assertion
was triggered.  Foreign tables can be part of a partition tree, and
again the assertion would have failed.  The simplest solution is just to
remove this assertion, so as we get the same failure as the
non-concurrent code path.

While on it, add a regression test in postgres_fdw for the concurrent
partition detach of a foreign table, as per a suggestion from Alexander
Lakhin.

Issue introduced in 71f4c8c.

Reported-by: Alexander Lakhin
Author: Michael Paquier, Alexander Lakhin
Reviewed-by: Peter Eisentraut, Kyotaro Horiguchi
Discussion: https://postgr.es/m/17339-a9e09aaf38a3457a@postgresql.org
Backpatch-through: 14

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -6485,6 +6485,9 @@ CREATE FOREIGN TABLE foreign_tbl (a int, b int)
   SERVER loopback OPTIONS (table_name 'child_tbl');
 CREATE TABLE parent_tbl (a int, b int) PARTITION BY RANGE(a);
 ALTER TABLE parent_tbl ATTACH PARTITION foreign_tbl FOR VALUES FROM (0) TO (100);
+-- Detach and re-attach once, to stress the concurrent detach case.
+ALTER TABLE parent_tbl DETACH PARTITION foreign_tbl CONCURRENTLY;
+ALTER TABLE parent_tbl ATTACH PARTITION foreign_tbl FOR VALUES FROM (0) TO (100);
 CREATE VIEW rw_view AS SELECT * FROM parent_tbl
   WHERE a < b WITH CHECK OPTION;
 \d+ rw_view

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -1490,6 +1490,9 @@ CREATE FOREIGN TABLE foreign_tbl (a int, b int)
 
 CREATE TABLE parent_tbl (a int, b int) PARTITION BY RANGE(a);
 ALTER TABLE parent_tbl ATTACH PARTITION foreign_tbl FOR VALUES FROM (0) TO (100);
+-- Detach and re-attach once, to stress the concurrent detach case.
+ALTER TABLE parent_tbl DETACH PARTITION foreign_tbl CONCURRENTLY;
+ALTER TABLE parent_tbl ATTACH PARTITION foreign_tbl FOR VALUES FROM (0) TO (100);
 
 CREATE VIEW rw_view AS SELECT * FROM parent_tbl
   WHERE a < b WITH CHECK OPTION;