tests: Bluetooth: ascs: Fix invalid NULL assignment

The iso_server shall be not set to NULL in mock_bt_iso_cleanup(), as
it's UUT responsibility to call bt_iso_server_unregister(). Cleaning
the iso_server in mock_bt_iso_cleanup() might hide bug in the code.

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>