fix UserModules.clear()

UserModules.clear() was wiping the entire cache, causing [this line](https://github.com/edk141/py3status/blob/c9cbccb27b9fd3d1e8e56b7adcc22325dbdac76b/py3status/__init__.py#L338) to throw exceptions because `class_name` wasn't in the cache dict, and ultimately making a lot of messages like this after a SIGUSR1:

```
Apr 29 14:19:21 keel py3status: injection failed ('notify.py')
Apr 29 14:20:35  py3status: last message repeated 13 times
```

My fix for this clears the cache at one level deep instead, which leaves it in a state where execute_classes() can still work.