Update README after submodule change

[pierwill]

Given the removal of the guide as a rust submodule in rust-lang/rust#81848, will the instructions at the bottom of this repo's README need updating?

[JohnTitor]

The submodule has been removed and added (for link-checking) several times. But given CI here is enough now and we haven't checked it since about a year ago, I think we could also remove the instructions, even right now.

[jieyouxu]

Triage: still outdated, linkcheck is currently disabled, r-l/r has periodical submodule bump PRs e.g. rust-lang/rust#131369

[jyn514]

we now use JOSH to synchronize with r-l/r, and that's documented in the readme (since #2183).