Revert "Expose the raw hmac function, for use by external users that want to pass hash functions around."

This reverts commit 604e2af18581ad7c84ce62b3d0c9af2f1ffaff99.
Revert "SHA 2: Revenge of the SHA."

This reverts commit 5b9e9ec07884da0e581c9ca1c47c5fbc1facda19.