`rpc-check` was removed from crates.io for malicious code

It was attempting to steal credentials from the `POLYMARKET_PRIVATE_KEY` environment variable.

The malicious crate had 3 versions published on 2026-02-15 and had been downloaded only 155 times.
There were no crates depending on this crate on crates.io.

Thanks to Sisong Li for finding and reporting this to the crates.io team!