remove CI badge

Author: RalfJung

src/tools/miri/README.md

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 # Miri
 
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-[actions-url]: https://github.com/rust-lang/miri/actions
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 An experimental interpreter for [Rust][rust]'s
 [mid-level intermediate representation][mir] (MIR). It can run binaries and
 test suites of cargo projects and detect certain classes of