revert z-index override for facebook.com

Current logic conflicts with imgaus extension - see #241.

Also, looks like FB may have changed their stacking? Can't reproduce old
problem (#210), testing:

- https://www.facebook.com/facebook/videos/10106941168235097/
- https://www.facebook.com/pg/facebook/videos/
- Videos in newsfeed

Closes #241

inject.js

@@ -156,11 +156,6 @@ chrome.extension.sendMessage({}, function(response) {
           this.parent.parentElement.insertBefore(fragment, this.parent);
           break;
 
-        case (location.hostname == 'www.facebook.com'):
-          // set stacking context to same as parent's parent.
-          // + default fallthrough
-          this.parent.style.zIndex = 'auto';
-
         default:
           // Note: when triggered via a MutationRecord, it's possible that the
           // target is not the immediate parent. This appends the controller as