Remove core::fmt::num::Decimal
Remove core::fmt::num::Decimal

Before ebf9e1aaf6, it was used for Display::fmt, but ebf9e1aaf6 replaced
that with a faster implementation, and nothing else uses it.

aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust