^
When you click on the lyrics field in the Tagging Pane, the editor will show up, with the lyrics contents highlighted. If you have several songs selected it will have [Varies] selected in the lyrics editor.
Either way, just paste in the expression, overwriting the highlighted lyrics, and it will get executed. Just close the editor after you paste it in and it will work.
I've just done a little more testing and figured out that my expression will badly break a Lyrics field that doesn't have the </script> key word in it. It will erase the Lyrics in that case! Ouch. If you can select only the ones that have the script part in them, great. If not, I've figured out another expression that accounts for either case. It's safe to use on regular lyrics fields and those that have your garbage you want removed.
=regex([Lyrics], /#(?:.*</script>)*(.+)#/,-1,0)[R1]
Brian.