[enh] move dictionaries, Erowid & IMDb out of general category

The general category is the category that is searched by default.
From a privacy standpoint it doesn't make sense to send all general
queries to specialized search engines that cannot deal with those
queries anyway.

searx/engines/duden.py

@@ -19,7 +19,7 @@ about = {
     "language": 'de',
 }
 
-categories = ['general', 'dictionaries']
+categories = ['dictionaries']
 paging = True
 
 # search-url