Remove two fields from `SubstFolder`.
Remove two fields from `SubstFolder`.

They're only used in error messages printed if there's an internal
compiler error, and the cost of maintaining them is high enough to show
up in profiles.

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