Clear prompt numbers in ClearOutput preprocessor

When choosing to clear all output in the notebook interface, it also clears all the prompt numbers. However, the `ClearOutput` preprocess does not currently clear prompt numbers -- only the outputs themselves. It seems like the behavior in these two places should be consistent, so this pull request modifies the `ClearOutput` preprocessor to also clear prompt numbers.
I'm sure you know the notebook format better than me, since you worked on the schema, but is it definitely correct to remove the key altogether, rather than setting it to None?