Remove LiipThemeBundle

As baggy theme was removed and material is the only remaining theme, we don't need a theme switched anymore.
So:
- move all `*.twig` files from the material theme folder to the root
- remove useless translations

src/Wallabag/CoreBundle/Controller/FeedController.php

@@ -144,7 +144,7 @@ class FeedController extends Controller
         }
 
         return $this->render(
-            '@WallabagCore/themes/common/Entry/entries.xml.twig',
+            '@WallabagCore/Entry/entries.xml.twig',
             [
                 'type' => 'tag',
                 'url' => $url,
@@ -228,7 +228,7 @@ class FeedController extends Controller
             }
         }
 
-        return $this->render('@WallabagCore/themes/common/Entry/entries.xml.twig', [
+        return $this->render('@WallabagCore/Entry/entries.xml.twig', [
             'type' => $type,
             'url' => $url,
             'entries' => $entries,