tests(ai_client): fix remaining ai_provider_unref() use-after-free in test_ai_parse_models_empty_json

The test called ai_provider_unref(provider) on a provider owned by the
hash table, then ai_client_shutdown() destroyed the hash table and
called ai_provider_unref() again on the dangling pointer.

Fix by removing the redundant unref call.

tests/unittests/test_ai_client.c

@@ -819,7 +819,6 @@ test_ai_parse_models_empty_json(void** state)
     assert_null(provider->models);
     assert_int_equal(0, g_list_length(provider->models));
 
-    ai_provider_unref(provider);
     ai_client_shutdown();
 }