refactor(types): remove dead catch-all clauses unreachable per success typing

lib/mv/mailer.ex

@@ -190,6 +190,4 @@ defmodule Mv.Mailer do
   defp valid_email?(email) when is_binary(email) do
     Regex.match?(@email_regex, String.trim(email))
   end
-
-  defp valid_email?(_), do: false
 end