Rollup merge of rust-lang#125880 - Zalathar:demangler, r=oli-obk

Remove `src/tools/rust-demangler`

`rust-demangler` is a small binary that reads a list of mangled symbols from stdin, demangles them (using the `rustc-demangle` library crate), and prints the demangled symbols to stdout.

It was added as part of the initial implementation of coverage instrumentation in 2020/2021, so that coverage tests could pass it to `llvm-cov --Xdemangler` when generating coverage reports. It has been largely untouched since then.

As of rust-lang#125816 it is no longer used by coverage tests, and has no remaining in-tree uses.

There is code in bootstrap to build and package the demangler, but it's unclear where the resulting binaries actually end up, or whether there's any reasonable way for `rustup` users to obtain them.

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For users needing a command-line demangler, `rustfilt` exists and is more actively maintained. It's also quite easy to use the `rustc-demangle` library to build a custom command-line demangler if necessary, with only a few lines of code.

The tool's name (`rust-demangler`) is easily confused with the name of the library crate `rustc-demangle`, so removing the tool will eliminate that confusion. There also doesn't appear to be much reason to use `rust-demangler` over `rustfilt`.

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This PR therefore removes the tool, and removes all of its associated code from bootstrap.

MCP filed: rust-lang/compiler-team#754