ML-180: remove backup download feature

Delete ArchiveController, its routes, the maintenance UI backup
button, and the test file. Management scripts and Litestream
supersede this functionality.

test/music_library_web/controllers/archive_controller_test.exs

@@ -1,17 +0,0 @@
-defmodule MusicLibraryWeb.ArchiveControllerTest do
-  use MusicLibraryWeb.ConnCase
-
-  describe "GET /backup" do
-    test "returns a sqlite database file", %{conn: conn} do
-      conn = get(conn, "/backup")
-
-      assert response(conn, 200)
-      assert get_resp_header(conn, "content-type") |> hd() =~ "application/x-sqlite3"
-
-      [content_disposition] = get_resp_header(conn, "content-disposition")
-      assert content_disposition =~ "attachment"
-      assert content_disposition =~ "music_library_"
-      assert content_disposition =~ ".db"
-    end
-  end
-end