test: remove dead anyio_backend fixture and unused pytest imports

The anyio_backend fixture is only consumed by pytest-anyio, which is not a
dependency of this project (we use pytest-asyncio with asyncio_mode=auto).
Also drop the unused pytest import from test_sanity.py.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/tests/conftest.py

@@ -1,8 +1 @@
 """Shared pytest fixtures for backend tests."""
-import asyncio
-import pytest
-
-
-@pytest.fixture
-def anyio_backend():
-    return "asyncio"

backend/tests/test_sanity.py

@@ -1,6 +1,5 @@
 """Sanity test to verify pytest infrastructure works."""
 import asyncio
-import pytest
 
 
 def test_sync_sanity():