Remove `HashStable` impls for `FileName` and `RealFileName`
Remove `HashStable` impls for `FileName` and `RealFileName`

These impls were unused, and incorrectly hashed the local
(non-remapped) path for `RealFileName::Remapped` (which would
break reproducible builds if these impls were used).

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