fix: remove merged JS files that were rewritten in TS (#5483)

Fix broken build from merging #5453 and #5463 to main in rapid sequence.

- #5453 converted some .es6 files to .ts
- #5463 renamed ALL .es6 files to .js
- Since #5453 was merged first, #5463's merge commit introduced duplicate .js files for the .es6 files that were already converted to .ts
- CI would catch this if it had been run again on #5463 again before merging that.

xref: https://chat.kobotoolbox.org/#narrow/stream/4-Kobo-Dev/topic/npm.20build.20failing.20on.20main.2002.2F03.2F25