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

config/config.exs

@@ -216,7 +216,6 @@ config :pleroma, :instance,
   allow_relay: true,
   public: true,
   quarantined_instances: [],
-  managed_config: true,
   static_dir: "instance/static/",
   allowed_post_formats: [
     "text/plain",

config/description.exs

@@ -764,12 +764,6 @@ config :pleroma, :config_description, [
           "*.quarantined.com"
         ]
       },
-      %{
-        key: :managed_config,
-        type: :boolean,
-        description:
-          "Whenether the config for pleroma-fe is configured in this config or in static/config.json"
-      },
       %{
         key: :static_dir,
         type: :string,