Remove code under GPDB_91_MERGE_FIXME in ATPrepAddColumn().

There are many tests which flow through this code path specifically in
alter_table.sql. Nothing exploded with the removal of the same and gpcheckcat
flagged nothing means its fine to delete the same.

src/backend/commands/tablecmds.c

@@ -6893,76 +6893,6 @@ ATPrepAddColumn(List **wqueue, Relation rel, bool recurse, bool recursing,
 			elog(ERROR, "unexpected ALTER TABLE subtype %d in ADD COLUMN command",
 				 cmd->subtype);
 	}
-
-	/* GPDB_91_MERGE_FIXME: I don't know if this is still needed or not.
-	 * Perhaps not, because of the new way OIDs are dispatched. We'll see when
-	 * we get to test this, I suppose.
-	 */
-#if 0
-	if (recurse)
-	{
-		/*
-		 * We are the master and the table has child(ren):
-		 * 		internally create and execute new AlterTableStmt(s) on child(ren)
-		 * 		before dispatching the original AlterTableStmt
-		 * This is to ensure that pg_constraint oid is consistent across segments for
-		 * 		ALTER TABLE ... ADD COLUMN ... CHECK ...
-		 */
-		if (Gp_role == GP_ROLE_DISPATCH)
-		{
-			List		*children;
-			ListCell	*lchild;
-
-			children = find_inheritance_children(RelationGetRelid(rel), NoLock);
-			DestReceiver *dest = None_Receiver;
-			foreach(lchild, children)
-			{
-				Oid 			childrelid = lfirst_oid(lchild);
-				Relation 		childrel;
-
-				RangeVar 		*rv;
-				AlterTableCmd 	*atc;
-				AlterTableStmt 	*ats;
-
-				if (childrelid == RelationGetRelid(rel))
-					continue;
-
-				childrel = heap_open(childrelid, AccessShareLock);
-				CheckTableNotInUse(childrel, "ALTER TABLE");
-
-				/* Recurse to child */
-				atc = copyObject(cmd);
-				if (cmd->subtype == AT_AddColumn || cmd->subtype == AT_AddColumnRecurse)
-					atc->subtype = AT_AddColumnRecurse;
-				else if (cmd->subtype == AT_AddOids)
-					atc->subtype = AT_AddOidsRecurse;
-				else
-					elog(ERROR, "unexpected ALTER TABLE subtype %d in ADD COLUMN command",
-						 cmd->subtype);
-
-				/* Child should see column as singly inherited */
-				((ColumnDef *) atc->def)->inhcount = 1;
-				((ColumnDef *) atc->def)->is_local = false;
-
-				rv = makeRangeVar(get_namespace_name(RelationGetNamespace(childrel)),
-								  get_rel_name(childrelid), -1);
-
-				ats = makeNode(AlterTableStmt);
-				ats->relation = rv;
-				ats->cmds = list_make1(atc);
-				ats->relkind = OBJECT_TABLE;
-
-				heap_close(childrel, NoLock);
-
-				ProcessUtility((Node *)ats,
-							   synthetic_sql,
-							   NULL,
-							   false, /* not top level */
-							   dest,
-							   NULL);
-			}
-		}
-#endif
 }
 
 static void