Remove RSAMD5 support from (*RRSIG).Verify (#1185)

AFAIK, the only way to get an RSAMD5 DNSKEY was to manually construct
one. This is ancient, just get rid of it.

The only remaining usage of md5 is in tsig.go. Hopefully that might be
removable as well.

dnssec.go

@@ -5,7 +5,6 @@ import (
  "crypto"
  "crypto/ecdsa"
  "crypto/elliptic"
- _ "crypto/md5"
  "crypto/rand"
  "crypto/rsa"
  _ "crypto/sha1"
@@ -437,7 +436,7 @@ func (rr *RRSIG) Verify(k *DNSKEY, rrset []RR) error {
  }
 
  switch rr.Algorithm {
- case RSASHA1, RSASHA1NSEC3SHA1, RSASHA256, RSASHA512, RSAMD5:
+ case RSASHA1, RSASHA1NSEC3SHA1, RSASHA256, RSASHA512:
  // TODO(mg): this can be done quicker, ie. cache the pubkey data somewhere??
  pubkey := k.publicKeyRSA() // Get the key
  if pubkey == nil {