syntax_ext: format: nest_level's are no more
syntax_ext: format: nest_level's are no more

`nest_level` is long dead since cac7a2053aba7be214d5e58e13867089638a8f50
(PR #14831), so is `check_positional_ok()`. Let's bid them farewell.

GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust