ninjabackend: remove useless and unused API bits in phony targets

I originally refactored this wrapper function in commit
dd2f1c4c57874a0efcd9f6d331985c408101c30d and at the time I made it take
the following call pattern:
```
self.create_phony_target(self.all_outputs, ..., implicit_outs=None)
```

I am not sure *why*. There are a couple problems here:
- When creating a phony target, there are never implicit outs and we never
  try to create them.
- It's invalid to call it without self.all_outputs as the first argument,
  so really we should just use that directly.

This made the function signature pointlessly longer and more
complicated.

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