Issue #20668: Remove tests.txt of test_asyncio

It's useless, tests are now discovered automatically.

Lib/test/test_asyncio/tests.txt

-test_asyncio.test_base_events
-test_asyncio.test_events
-test_asyncio.test_futures
-test_asyncio.test_locks
-test_asyncio.test_proactor_events
-test_asyncio.test_queues
-test_asyncio.test_selector_events
-test_asyncio.test_streams
-test_asyncio.test_subprocess
-test_asyncio.test_tasks
-test_asyncio.test_transports
-test_asyncio.test_unix_events
-test_asyncio.test_windows_events
-test_asyncio.test_windows_utils