web/endpoint: drop unused function load_from_system_env

It’s not called anywhere in out code and
also doesn’t appear to be part of Phoenix.Endpoint’s interface

lib/pleroma/web/endpoint.ex

@@ -168,18 +168,6 @@ defmodule Pleroma.Web.Endpoint do
 
   plug(Pleroma.Web.Router)
 
-  @doc """
-  Dynamically loads configuration from the system environment
-  on startup.
-
-  It receives the endpoint configuration from the config files
-  and must return the updated configuration.
-  """
-  def load_from_system_env(config) do
-    port = System.get_env("PORT") || raise "expected the PORT environment variable to be set"
-    {:ok, Keyword.put(config, :http, [:inet6, port: port])}
-  end
-
   def websocket_url do
     String.replace_leading(url(), "http", "ws")
   end