Remove the `web-streams-polyfill` Babel-exclusion in `gulpfile.js` (PR 14560 follow-up)
Remove the `web-streams-polyfill` Babel-exclusion in `gulpfile.js` (PR 14560 follow-up)

Given that we're no longer, after PR 14560, bundling the `web-streams-polyfill`-code in the `legacy`-builds we shouldn't need to exclude it from Babel now.

Shoobx/pdf.js,Snuffleupagus/pdf.js,mozilla/pdf.js,nawawi/pdf.js,nawawi/pdf.js,Snuffleupagus/pdf.js,timvandermeij/pdf.js,showpad/mozilla-pdf.js,timvandermeij/pdf.js,nawawi/pdf.js,showpad/mozilla-pdf.js,Shoobx/pdf.js,showpad/mozilla-pdf.js,timvandermeij/pdf.js,mozilla/pdf.js,Snuffleupagus/pdf.js,Shoobx/pdf.js,mozilla/pdf.js