Remove temporary raw_response argument

We added a new argument to `client_request.get` and
`client_request.post` to specify that it should return a raw `Response`
object rather than an instance of `BeautifulSoup`.

This is useful because sometimes we need to look at stuff like the
response headers.

However it turns out we already have a separate method for this, so
rather than invent something new I think it’s better to stick with the
thing we already have.

tests/app/main/views/test_dashboard.py

@@ -475,10 +475,9 @@ def test_download_inbox_strips_formulae(
             }]
         },
     )
-    response = client_request.get(
+    response = client_request.get_response(
         'main.inbox_download',
         service_id=SERVICE_ONE_ID,
-        _raw_response=True,
     )
     assert expected_cell in response.get_data(as_text=True).split('\r\n')[1]