Remove the undocumented PDFJS.enableStats option

In order to simplify things, the undocumented `enableStats` option was removed and `pdfBug` is now instead used to enabled general debugging *and* page request/rendering stats.
Considering that in the default viewer the `stats` was only used when debugging was also enabled, this simplification (code wise) definitely seem worthwhile to me.

src/display/dom_utils.js

@@ -359,8 +359,6 @@ function getDefaultSetting(id) {
       return globalSettings ? globalSettings.maxImageSize : -1;
     case 'isEvalSupported':
       return globalSettings ? globalSettings.isEvalSupported : true;
-    case 'enableStats':
-      return !!(globalSettings && globalSettings.enableStats);
     default:
       throw new Error('Unknown default setting: ' + id);
   }