Rollup merge of #57652 - mark-i-m:remove-old, r=nikomatsakis

Update/remove some old readmes

r? @nikomatsakis

cc #48478

There are a bunch of READMEs with content that I would like to see a final decision made on:
- https://github.com/rust-lang/rust/tree/master/src/librustc/ty/query
- https://github.com/rust-lang/rust/tree/master/src/librustc/dep_graph
- https://github.com/rust-lang/rust/blob/master/src/librustc/infer/region_constraints
- https://github.com/rust-lang/rust/tree/master/src/librustc/infer/higher_ranked
- https://github.com/rust-lang/rust/tree/master/src/librustc/infer/lexical_region_resolve
- https://github.com/rust-lang/rust/blob/master/src/librustc_borrowck/borrowck

It's not clear how useful or obsolete any of these are. I would really appreciate if the appropriate domain experts for each of these could respond with one of (a) delete it, (b) wait for system to be remove, or (c) move it to rustc-guide. @nikomatsakis do you know who to ping for any of these (sorry, I suspect many of them are you)?

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