fixit upgrade makes incomplete fix, second run results in metadata error

```
from fixit import CstLintRule

class FooRule(CstLintRule):
...
```

```
Traceback (most recent call last):
File ../fixit/upgrade/remove_rule_suffix.py", line 58, in visit_ClassDef
qname = metadata.pop().name
KeyError: 'pop from an empty set'
```