Fix another Assert issue exposed by CLOBBER_CACHE_ALWAYS.

plpgsql's exec_stmt_execsql was Assert'ing that a CachedPlanSource was
is_valid immediately after exec_prepare_plan.  The risk factor in this case
is that after building the prepared statement, exec_prepare_plan calls
exec_simple_check_plan, which might try to generate a generic plan --- and
with CLOBBER_CACHE_ALWAYS or other unusual causes of invalidation, that
could result in an invalidation.  However, that path could only be taken
for a SELECT query, for which we need not set mod_stmt.  So in this case
I think it's best to just remove the Assert; it's okay to look at a
slightly-stale querytree for what we need here.  Per buildfarm testing.

src/pl/plpgsql/src/pl_exec.c

@@ -3027,7 +3027,6 @@ exec_stmt_execsql(PLpgSQL_execstate *estate,
 			CachedPlanSource *plansource = (CachedPlanSource *) lfirst(l);
 			ListCell   *l2;
 
-			Assert(plansource->is_valid);
 			foreach(l2, plansource->query_list)
 			{
 				Query  *q = (Query *) lfirst(l2);