remove now-unused import_module import

The last import_module() call site went away when
`asyncio = import_module("asyncio")` became a plain import, leaving
the import orphaned and tripping ruff's F401 in the lint job.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: graingert

Lib/test/test_asyncgen.py

@@ -5,7 +5,6 @@
 import unittest
 import contextlib
 
-from test.support.import_helper import import_module
 from test.support import gc_collect, requires_working_socket, async_yield as _async_yield