Don't show download button for Multi-Site

Looking at the [view method](https://github.com/concrete5/concrete5/blob/8c99078da2e5cddfe72767d23ddcf28b091e1516/web/concrete/core/controllers/single_pages/dashboard/system/backup_restore/update.php#L32) in the Dashboard controller to update concrete5, the logic is that a Multi-Sites cannot download updates. This is also enforced in [download_update method](https://github.com/concrete5/concrete5/blob/8c99078da2e5cddfe72767d23ddcf28b091e1516/web/concrete/core/controllers/single_pages/dashboard/system/backup_restore/update.php#L73).

However the check for `$showDownloadBox` got somehow lost along bad8e456af6b6b70c2c45c302c135a62192992ef and 632dcde05b18ef79bc4dc7e1017be463356ab574.

Ping @KorvinSzanto