Remove invalid comment

The TUF 1.0 Spec does not specify how libraries should handle private keys,
so the PrivateKeyValue.Private field is irrelevant for the TUF-1.0
migration.

Change-Id: I69f32fb6f30ea2156abdffe4c6e48bf805295c56
diff --git a/sign/keys.go b/sign/keys.go
index 0f93e1f..1ef0ee3 100644
--- a/sign/keys.go
+++ b/sign/keys.go
@@ -17,9 +17,6 @@
 
 type PrivateKeyValue struct {
 	Public data.HexBytes `json:"public"`
-
-	// FIXME(TUF-0.9) This is removed in TUF 1.0, keeping around for
-	// compatibility with TUF 0.9.
 	Private data.HexBytes `json:"private"`
 }