[fix] make reddit only in social media category avail.

fix https://github.com/searxng/searxng/issues/470

searx/engines/reddit.py

@@ -18,7 +18,7 @@ about = {
 }
 
 # engine dependent config
-categories = ['general', 'images', 'news', 'social media']
+categories = ['social media']
 page_size = 25
 
 # search-url