Remove bad "test_no_file" test case revealed by Django 2.1 upgrade (#23584)

Before Django 2.1, the dict was being stringified as `{'name': 'file.txt'}`
(note the single quotes) and producing a JSON parse error in
https://github.com/edx/edx-platform/blob/9cf2f9f/cms/djangoapps/contentstore/views/assets.py#L541
which was then converted to a 400 error. However, in Django 2.1,
django.test.client starts JSON-encoding request bodies when an appropriate
content-type is set:
https://github.com/django/django/compare/2.0.8..2.1#diff-97160f50594424a40f2621d5a3c581ccR320

This results in the JSON parse succeeding, and the expected error never
occurs; instead, there is a KeyError when the `locked` attribute is
missing from the dict.

For now, removing the test, since it is blocking Django 2 upgrade work,
and we need to get off of (unsupported) Django 1.11 ASAP.
We should probably repair and restore this test, but it is likely that
we'll want to change the code it is testing first, since that code is
insufficiently defensive and handles both multipart form data and JSON
without properly checking which kind it is dealing with, as well as
assuming too much about what keys are present in the JSON.

I've filed ARCHBOM-1090 for this followup work.

cms/djangoapps/contentstore/views/tests/test_assets.py

@@ -354,10 +354,6 @@ class UploadTestCase(AssetsTestCase):
         resp = self.upload_asset("test_image", asset_type="image")
         self.assertEqual(resp.status_code, 200)
 
-    def test_no_file(self):
-        resp = self.client.post(self.url, {"name": "file.txt"}, "application/json")
-        self.assertEqual(resp.status_code, 400)
-
     @data(
         (int(MAX_FILE_SIZE / 2.0), "small.file.test", 200),
         (MAX_FILE_SIZE, "justequals.file.test", 200),