Remove unnecessary code

The code to delete an attribute that is really a field in the internal
document became unnecessary with the changes to `Model.__setattr__()` in
5f1924c. The unnecessary code is being removed from `__delattr__()`.
Clean up

Because `pymongo_supports_mongoclient` is supposed to be set to `True`
only when no exception has occurred, it's not being handled inside the
`else` associated with the try/except rather than directly in the try.
Use the ignored context manager

The `ignored` context manager was added in 658c508. These are the
locations that could benefit from it. They're being updated to use it
instead of the try/except/pass style `ignored()` is meant to replace.