nixos/test-driver: fix return value of subtest function

Mypy since version 1.10.0 complains about this:

> test_driver/driver.py:109: error: No return value expected  [return-value]

nixos/lib/test-driver/test_driver/driver.py

@@ -106,7 +106,6 @@ class Driver:
         with self.logger.subtest(name):
             try:
                 yield
-                return True
             except Exception as e:
                 self.logger.error(f'Test "{name}" failed with error: "{e}"')
                 raise e