Remove incorrect compose_len from rrsig (#313)

ProtoRrsig is used to produce the data that is signed. According to
RFC 4034, this serialization begins with the first rdata field which is
type_covered, however ProtoRrsig is inserting a length at the beginning. As
such, signatures produced by SortedRecords::sign() are invalid. Fix this by
removing that length from the serialization.

src/rdata/dnssec.rs

@@ -505,22 +505,10 @@ impl<Name: ToName> ProtoRrsig<Name> {
         self.signer_name.compose_canonical(target)
     }
 
-    fn compose_len(&self) -> u16 {
-        Rtype::COMPOSE_LEN
-            + SecAlg::COMPOSE_LEN
-            + u8::COMPOSE_LEN
-            + u32::COMPOSE_LEN
-            + Timestamp::COMPOSE_LEN
-            + Timestamp::COMPOSE_LEN
-            + u16::COMPOSE_LEN
-            + self.signer_name.compose_len()
-    }
-
     fn compose_head<Target: Composer + ?Sized>(
         &self,
         target: &mut Target,
     ) -> Result<(), Target::AppendError> {
-        self.compose_len().compose(target)?;
         self.type_covered.compose(target)?;
         self.algorithm.compose(target)?;
         self.labels.compose(target)?;