Remove no-op pull_var_clause flag

Hi.

While merging PostgreSQL to our (Shardman) fork, I've noticed that it 
uses the same pull_var_clause() flag as we do (coming from 
https://www.postgresql.org/message-id/CAFjFpRc8ZoDm0%2Bzhx%2BMckwGyEqkOzWcpVqbvjaxwdGarZSNrmA%40mail.gmail.com). 
I was a bit surprised, the patch hasn't landed to master. But flag 
somehow slipped in (but only as no-op). The attached patch removes 
useless flag.

-- 
Best regards,
Alexander Pyhalov,
Postgres Professional