[twitter] don't cache results of 'user_by_screen_name()'

A 'keyarg=1' argument to the memcache decorator would have worked as
well, but keeping the user object in memory isn't useful for the vast
majority of use cases and only wastes space.

(closes #817)

gallery_dl/extractor/twitter.py

@@ -10,7 +10,7 @@
 
 from .common import Extractor, Message
 from .. import text, exception
-from ..cache import cache, memcache
+from ..cache import cache
 import hashlib
 import time
 
@@ -441,7 +441,6 @@ class TwitterAPI():
         endpoint = "2/timeline/bookmark.json"
         return self._pagination(endpoint)
 
-    @memcache()
     def user_by_screen_name(self, screen_name):
         endpoint = "graphql/-xfUfZsnR_zqjFd-IfrN5A/UserByScreenName"
         params = {