Remove --cfg dox from rustdoc.rs
Remove --cfg dox from rustdoc.rs

This was added in https://github.com/rust-lang/rust/pull/53076 because
several dependencies were using `cfg(dox)` instead of `cfg(rustdoc)`.
I ran `rg 'cfg\(dox\)'` on the source tree with no matches, so I think
this is now safe to remove.

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