This fixes https://github.com/prometheus/client_golang/issues/74 .
IT CHANGES THE FINGERPRINTING AND THEREFORE INVALIDATES EACH AND EVERY
PERSISTED FINGERPRINT (I.E. YOU HAVE TO WIPE THE STORAGE TO USE THIS).
This commit removes the LabelValuesToSignature function as it is used
nowhere (and broken in the way it is implemented right now).
Also, remove one more golint warning.