Fix dialyzer error in object fetcher: Add proper guard clause for check_crossdomain_redirect/2

Also remove unnecessary and incorrect usage of Mix.env()

lib/pleroma/object/fetcher.ex

@@ -19,8 +19,6 @@ defmodule Pleroma.Object.Fetcher do
   require Logger
   require Pleroma.Constants
 
-  @mix_env Mix.env()
-
   @spec reinject_object(struct(), map()) :: {:ok, Object.t()} | {:error, any()}
   defp reinject_object(%Object{data: %{}} = object, new_data) do
     Logger.debug("Reinjecting object #{new_data["id"]}")
@@ -178,14 +176,9 @@ defmodule Pleroma.Object.Fetcher do
   def fetch_and_contain_remote_object_from_id(_id),
     do: {:error, "id must be a string"}
 
-  defp check_crossdomain_redirect(final_host, original_url)
-
-  # Handle the common case in tests where responses don't include URLs
-  if @mix_env == :test do
-    defp check_crossdomain_redirect(nil, _) do
+  defp check_crossdomain_redirect(final_host, _original_url) when is_nil(final_host) do
     {:cross_domain_redirect, false}
   end
-  end
 
   defp check_crossdomain_redirect(final_host, original_url) do
     {:cross_domain_redirect, final_host != URI.parse(original_url).host}