Avoid using lcons and list_delete_first in plan_union_children()

[Hou, Zhijie]

Hi,

In function plan_union_children(),  
I found the lcons and list_delete_first here is easy to be replaced by lappend 
and list_delete_last.

And I also found a previous commit do similar thing, so I try to improve this 
one.

Previous commit:
d97b714a219959a50f9e7b37ded674f5132f93f3

Best regards,
houzj

0001-Avoid-using-lcons-and-list_delete_first.patch
Description: 0001-Avoid-using-lcons-and-list_delete_first.patch