remove dead test file

tests/unit/backup.bats

@@ -1,20 +0,0 @@
-#!/usr/bin/env bats
-
-# load test_helper
-
-# setup() {
-#   create_app
-#   backup_file=$(mktemp /tmp/backup.XXXXXX)
-#   rm $backup_file
-# }
-
-# teardown() {
-#   destroy_app
-# }
-
-# @test "(backup) backup:export" {
-#   run dokku backup:export $backup_file
-#   echo "output: "$output
-#   echo "status: "$status
-#   assert_success
-# }