Fix startup error on windows (#1128)

Fixes https://github.com/simonw/datasette/issues/1094

This import isn't used at all, and causes error on startup on Windows.

datasette/utils/asgi.py

@@ -1,5 +1,4 @@
 import json
-from os import EX_CANTCREAT
 from datasette.utils import MultiParams
 from mimetypes import guess_type
 from urllib.parse import parse_qs, urlunparse, parse_qsl