Re: Retire has_multiple_baserels()

I used the following patch to double check that nothing was missed:

```
--- a/src/backend/optimizer/path/allpaths.c
+++ b/src/backend/optimizer/path/allpaths.c
@@ -2207,8 +2207,13 @@ has_multiple_baserels(PlannerInfo *root)
                /* ignore RTEs that are "other rels" */
                if (brel->reloptkind == RELOPT_BASEREL)
                        if (++num_base_rels > 1)
+                       {
+
Assert(bms_membership(root->all_baserels) == BMS_MULTIPLE);
                                return true;
+                       }
        }
+
+       Assert(bms_membership(root->all_baserels) != BMS_MULTIPLE);
        return false;
 }
```

It wasn't. The patch LGTM.