slack_import: Remove obsolete SlackImportAttachment placeholder.

This was introduced in f4ad464d82, and
incompletely removed in e037c2f93e649c28a71c02559b5ae7a3333f42a8; here
we finish removing it.

zerver/data_import/slack.py

@@ -1094,8 +1094,6 @@ def get_attachment_path_and_content(fileinfo: ZerverFieldsT, realm_id: int) -> T
     s3_path = "/".join(
         [
             str(realm_id),
-            "SlackImportAttachment",  # This is a special placeholder which should be kept
-            # in sync with 'exports.py' function 'import_message_data'
             format(random.randint(0, 255), "x"),
             secrets.token_urlsafe(18),
             sanitize_name(fileinfo["name"]),

zerver/tests/test_slack_importer.py

@@ -1137,7 +1137,6 @@ class SlackImporter(ZulipTestCase):
         self.assertEqual(len(uploads_list), 1)
 
         image_path = zerver_attachment[0]["path_id"]
-        self.assertIn("/SlackImportAttachment/", image_path)
         expected_content = f"[Apple](/user_uploads/{image_path})\n[banana](example.com/banana.zip)"
         self.assertEqual(info["content"], expected_content)