Remove calls to deprecated request.is_ajax()
Remove calls to deprecated request.is_ajax()

Since Django 3.1, the method request.is_ajax() is deprecated and will be
removed in a future version. Using Django 3.1 with django-debug-toolbar
results in the warning:

    django-debug-toolbar/debug_toolbar/middleware.py:80: RemovedInDjango40Warning: request.is_ajax() is deprecated. See Django 3.1 release notes for more details about this deprecation.
      request.is_ajax(),

The calls were removed with no replacements as these check are
unnecessary anyway. If the response is not HTML (e.g. JSON) then the
check `or content_type not in _HTML_TYPES` will evaluate as false and
the toolbar will not be included in the response. If the response is an
HTML fragment, it will not contain an `</body>` element (the
INSERT_BEFORE pattern) and so will also not be included in the response.

Fixes #1368

spookylukey/django-debug-toolbar,jazzband/django-debug-toolbar,django-debug-toolbar/django-debug-toolbar,tim-schilling/django-debug-toolbar,jazzband/django-debug-toolbar,tim-schilling/django-debug-toolbar,django-debug-toolbar/django-debug-toolbar,spookylukey/django-debug-toolbar,jazzband/django-debug-toolbar,tim-schilling/django-debug-toolbar,django-debug-toolbar/django-debug-toolbar,spookylukey/django-debug-toolbar