Fix unused variable warning

Author: josevalim

lib/logger/lib/logger/handler.ex

@@ -90,7 +90,6 @@ defmodule Logger.Handler do
             :ok
 
           {message, %{gl: gl} = metadata} ->
-            timestamp = Map.get_lazy(metadata, :time, fn -> :os.system_time(:microsecond) end)
             metadata = erlang_metadata_to_elixir_metadata(metadata)
             %{truncate: truncate, utc_log: utc_log?} = config