Remove release_env

While taking a final look at instance.gen before releasing I noticed
that the release_env task outputs messages in broken english. Upon
further inspection it seems to have even more severe issues which, in
my opinion, warrant it's at least temporary removal:
- We do not explain what it actually does, anywhere. Neither the task
 docs nor instance.gen, nor installation instructions.
- It does not respect FHS on OTP releases (uses /opt/pleroma/config even
 though we store the config in /etc/pleroma/config.exs).
- It doesn't work on OTP releases, which is the main reason it exists.
Neither systemd nor openrc service files for OTP include it.
- It is not mentioned in install guides other than the ones for Debian
and OTP releases.

test/mix/tasks/pleroma/instance_test.exs

@@ -5,8 +5,6 @@
 defmodule Mix.Tasks.Pleroma.InstanceTest do
   use ExUnit.Case
 
-  @release_env_file "./test/pleroma.test.env"
-
   setup do
     File.mkdir_p!(tmp_path())
 
@@ -18,8 +16,6 @@ defmodule Mix.Tasks.Pleroma.InstanceTest do
         File.rm_rf(Path.join(static_dir, "robots.txt"))
       end
 
-      if File.exists?(@release_env_file), do: File.rm_rf(@release_env_file)
-
       Pleroma.Config.put([:instance, :static_dir], static_dir)
     end)
 
@@ -73,9 +69,7 @@ defmodule Mix.Tasks.Pleroma.InstanceTest do
         "--dedupe-uploads",
         "n",
         "--anonymize-uploads",
-        "n",
-        "--release-env-file",
-        @release_env_file
+        "n"
       ])
     end
 
@@ -97,9 +91,6 @@ defmodule Mix.Tasks.Pleroma.InstanceTest do
     assert generated_config =~ "filters: [Pleroma.Upload.Filter.ExifTool]"
     assert File.read!(tmp_path() <> "setup.psql") == generated_setup_psql()
     assert File.exists?(Path.expand("./test/instance/static/robots.txt"))
-    assert File.exists?(@release_env_file)
-
-    assert File.read!(@release_env_file) =~ ~r/^RELEASE_COOKIE=.*/
   end
 
   defp generated_setup_psql do

test/mix/tasks/pleroma/release_env_test.exs

@@ -1,30 +0,0 @@
-# Pleroma: A lightweight social networking server
-# Copyright © 2017-2020 Pleroma Authors <https://pleroma.social/>
-# SPDX-License-Identifier: AGPL-3.0-only
-
-defmodule Mix.Tasks.Pleroma.ReleaseEnvTest do
-  use ExUnit.Case
-  import ExUnit.CaptureIO, only: [capture_io: 1]
-
-  @path "config/pleroma.test.env"
-
-  def do_clean do
-    if File.exists?(@path) do
-      File.rm_rf(@path)
-    end
-  end
-
-  setup do
-    do_clean()
-    on_exit(fn -> do_clean() end)
-    :ok
-  end
-
-  test "generate pleroma.env" do
-    assert capture_io(fn ->
-             Mix.Tasks.Pleroma.ReleaseEnv.run(["gen", "--path", @path, "--force"])
-           end) =~ "The file generated"
-
-    assert File.read!(@path) =~ "RELEASE_COOKIE="
-  end
-end