[api-major] Remove the PDFJS_NEXT option

Nothing uses this option anymore, so setting it is a no-op now. We can
safely remove it.

Use `SKIP_BABEL` (instead of `PDFJS_NEXT`) now if you want to skip Babel
translation for a build.

src/display/dom_utils.js

@@ -443,8 +443,6 @@ function getDefaultSetting(id) {
       return globalSettings ? globalSettings.externalLinkRel : DEFAULT_LINK_REL;
     case 'enableStats':
       return !!(globalSettings && globalSettings.enableStats);
-    case 'pdfjsNext':
-      return !!(globalSettings && globalSettings.pdfjsNext);
     default:
       throw new Error('Unknown default setting: ' + id);
   }