Remove non-relative Seek calls from all functions.

Previously we would seek to the beginning of the io.ReadSeeker. It is much more
flexible to assume that the caller passes the ReadSeeker in an appropriate state.

flac.go

@@ -27,11 +27,6 @@ const (
 // ReadFLACTags reads FLAC metadata from the io.ReadSeeker, returning the resulting
 // metadata in a Metadata implementation, or non-nil error if there was a problem.
 func ReadFLACTags(r io.ReadSeeker) (Metadata, error) {
-	_, err := r.Seek(0, os.SEEK_SET)
-	if err != nil {
-		return nil, err
-	}
-
 	flac, err := readString(r, 4)
 	if err != nil {
 		return nil, err