fix(file_state): drop the module-level forget_task wrapper

The wrapper (and its __all__ entry) had no production caller: the only
release path, tools/file_tools.py::clear_file_ops_cache, already goes
through file_state.get_registry().forget_task(). It existed solely for
the new registry test, which now calls get_registry().forget_task()
directly, the same path production takes.

tests/tools/test_file_state_registry.py

@@ -182,7 +182,7 @@ class FileStateRegistryUnitTests(unittest.TestCase):
         registry = file_state.get_registry()
         self.assertEqual(registry._last_writer[p][0], "cron:JOB:run1")
 
-        file_state.forget_task("cron:JOB:run1")
+        registry.forget_task("cron:JOB:run1")
 
         self.assertNotIn(p, registry._last_writer)
         self.assertIsNone(file_state.check_stale("cron:JOB:run2", p))

tools/file_state.py

@@ -249,10 +249,6 @@ def known_reads(task_id: str) -> List[str]:
     return _registry.known_reads(task_id)
 
 
-def forget_task(task_id: str) -> None:
-    _registry.forget_task(task_id)
-
-
 __all__ = [
     "FileStateRegistry",
     "get_registry",
@@ -261,5 +257,4 @@ __all__ = [
     "check_stale",
     "lock_path",
     "writes_since",
-    "known_reads",
-    "forget_task"]
+    "known_reads"]