remove link to removed readme

Author: mark-i-m

src/README.md

@@ -8,7 +8,6 @@ For more information on how various parts of the compiler work, see the [rustc g
 There is also useful content in the following READMEs, which are gradually being moved over to the guide:
 - 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