Removed doc reference to removed HttpResponse.mimetype.

docs/ref/request-response.txt

@@ -661,8 +661,6 @@ Methods
     specified, it is formed by the :setting:`DEFAULT_CONTENT_TYPE` and
     :setting:`DEFAULT_CHARSET` settings, by default: "`text/html; charset=utf-8`".
 
-    Historically, this parameter was called ``mimetype`` (now deprecated).
-
     ``status`` is the `HTTP status code`_ for the response.
 
     ``reason`` is the HTTP response phrase. If not provided, a default phrase