Remove use of deprecated request.is_ajax()

The method was deprecated in upstream commit
django/django@e348ab0.

Refs #1032

Author: jdufresne

debug_toolbar/middleware.py

@@ -45,10 +45,9 @@ def __init__(self, get_response):
         self.get_response = get_response
 
     def __call__(self, request):
-        # Decide whether the toolbar is active for this request. Don't render
-        # the toolbar during AJAX requests.
+        # Decide whether the toolbar is active for this request.
         show_toolbar = get_show_toolbar()
-        if not show_toolbar(request) or request.is_ajax():
+        if not show_toolbar(request):
             return self.get_response(request)
 
         toolbar = DebugToolbar(request, self.get_response)