[remove] remove searchcode_doc and twitter

* twitter: the API has changed. the engine needs to rewritten.
* searchcode_doc: the API about documentation doesn't exist anymore.

searx/settings.yml

@@ -698,10 +698,6 @@ engines:
     engine : stackoverflow
     shortcut : st
 
-  - name : searchcode doc
-    engine : searchcode_doc
-    shortcut : scd
-
   - name : searchcode code
     engine : searchcode_code
     shortcut : scc
@@ -768,10 +764,6 @@ engines:
     categories : onions
     shortcut : tch
 
-  - name : twitter
-    engine : twitter
-    shortcut : tw
-
 # maybe in a fun category
 #  - name : uncyclopedia
 #    engine : mediawiki