Remove SWAL from htdocs/vendor

[driusan]

We currently have 2 versions of swal used in LORIS: sweetalert2, managed by npm, and sweetalert, which is placed in the htdocs/vendor/js/sweetalert directory. We should migrate all usage to be the sweetalert2 version and remove the old vendored code.

[laemtl]

@haoweiqiu See #6159 for a list of suggestions to completely address this issue.