127373822e
This was already only enabled in debug_assertions builds. Generally, it seems like most use cases that would use this could also use the -Zself-profile flag which also tracks cache hits (in all builds), and so the extra cfg's and such are not really necessary. This is largely just a small cleanup though, which primarily is intended to make other changes easier by avoiding the need to deal with this field.
For more information about how the query system works, see the rustc dev guide.