Remove :managed_config option.

In practice, it was already removed half a year ago, but the description
 and cheatsheet entries were still there.

The migration intentionally does not use ConfigDB.get_by_params, since
this will break migration code as soon as we add a new field is added
 to ConfigDB.

Closes #2086

priv/repo/migrations/20200910113106_remove_managed_config_from_db.exs

@@ -0,0 +1,27 @@
+defmodule Pleroma.Repo.Migrations.RemoveManagedConfigFromDb do
+  use Ecto.Migration
+  import Ecto.Query
+  alias Pleroma.ConfigDB
+  alias Pleroma.Repo
+
+  def up do
+    config_entry =
+      from(c in ConfigDB,
+        select: [:id, :value],
+        where: c.group == ^:pleroma and c.key == ^:instance
+      )
+      |> Repo.one()
+
+    if config_entry do
+      {_, value} = Keyword.pop(config_entry.value, :managed_config)
+
+      config_entry
+      |> Ecto.Changeset.change(value: value)
+      |> Repo.update()
+    end
+  end
+
+  def down do
+    :ok
+  end
+end