Remove core::fmt::num::Decimal

[glandium]

Before ebf9e1a, it was used for Display::fmt, but ebf9e1a replaced
that with a faster implementation, and nothing else uses it.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @aturon (or someone else) soon.

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[aturon]

@bors: r+ rollup

[bors]

📌 Commit 9e62681 has been approved by aturon