Remove unmatched function per analyzer warning

lib/gc_index_relay/nostr/filter.ex

@@ -144,8 +144,6 @@ defmodule GcIndexRelay.Nostr.Filter do
     end
   end
 
-  defp validate_tags(_), do: {:error, "Filter tags must be an object"}
-
   @spec validate_tag_keys(map()) :: :ok | {:error, String.t()}
   defp validate_tag_keys(tags) do
     # Inline validation logic for better locality of behavior