Remove "Keys: " from create auth key output

This is based on the premis that "the user know what command they
executed" and therefor know that the output is the key.

This makes the command a lot more useful in scripts.

cmd/headscale/cli/preauthkeys.go

@@ -116,6 +116,6 @@ var createPreAuthKeyCmd = &cobra.Command{
 			fmt.Println(err)
 			return
 		}
-		fmt.Printf("Key: %s\n", k.Key)
+		fmt.Printf("%s\n", k.Key)
 	},
 }