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

src/backend/commands/tablecmds.c

@@ -15013,8 +15013,6 @@ MarkInheritDetached(Relation child_rel, Relation parent_rel)
 	HeapTuple	inheritsTuple;
 	bool		found = false;
 
-	Assert(child_rel->rd_rel->relkind == RELKIND_RELATION ||
-		   child_rel->rd_rel->relkind == RELKIND_PARTITIONED_TABLE);
 	Assert(parent_rel->rd_rel->relkind == RELKIND_PARTITIONED_TABLE);
 
 	/*