settings: Remove unused slack_importer_test_settings.

Commit 3ebd30120c (#8345) removed the
corresponding test.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zproject/slack_importer_test_settings.py

@@ -1,10 +0,0 @@
-from .settings import *
-
-DATABASES["default"] = {
-    "NAME": "zulip_slack_importer_test",
-    "USER": "zulip_test",
-    "PASSWORD": LOCAL_DATABASE_PASSWORD,
-    "HOST": "localhost",
-    "SCHEMA": "zulip",
-    "ENGINE": "django.db.backends.postgresql",
-}