Remove the preallocation of signatures slice. (#869)

This was making codeql upset. I don't think there's a real issue, but better safe
than sorry.

Signed-off-by: Dan Lorenc <lorenc.d@gmail.com>

pkg/cosign/tuf/policy.go

@@ -174,7 +174,7 @@ func (s *Signed) AddOrUpdateSignature(signature Signature) error {
 	if !root.ValidKey(signature.KeyID, "root") {
 		return errors.New("invalid root key")
 	}
-	signatures := make([]Signature, 0, len(s.Signatures)+1)
+	signatures := []Signature{}
 	for _, sig := range s.Signatures {
 		if sig.KeyID != signature.KeyID {
 			signatures = append(signatures, sig)