Remove unnecessary relabel stripping

Hi hackers,

Per discussion in [1], we don't need to strip relabel for the expr
explicitly before calling pull_varnos() to retrieve all mentioned
relids. pull_varnos() would recurse into T_RelabelType nodes.

Add a patch to remove that and simplify the code a bit.

[1]
https://www.postgresql.org/message-id/flat/CAMbWs48HF9f%3Dg%2BjSmmYBnWub9%2BWyg5Xh-FoqAnvqAspue5ypAw%40mail.gmail.com#b6e77e4c1ae67e2c5d97dce830b58037

Thanks
Richard