scripts: runners: nios2 runner has been dropped

Remove `nios2` from list of available runners as it's just been dropped
with commit 5fe84d5

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

Author: kartben

scripts/west_commands/runners/__init__.py

@@ -44,7 +44,6 @@ def _import_runner_module(runner_name):
     'minichlink',
     'misc',
     'native',
-    'nios2',
     'nrfjprog',
     'nrfutil',
     'nsim',