Remove unused operator.py module

[spencerkclark]

Closes #174

[spencerahill]

Awesome, thanks! Let's also remove its section in the API reference docs page. And I'd do a grep through the whole package after that for 'operator' to make sure there are no other dangling references.

Once you've done those, go ahead and merge. This doesn't need a what's new entry.

[spencerahill]

Actually let's just add a what's new entry under Bug fixes, which can just be

- Remove obsolete operator.py module (fixes :issue:`174` via :pull:`175`).  By `Spencer Clark <https://github.com/spencerkclark>`_.

[spencerkclark]

Let's also remove its section in the API reference docs page.

Ah good catch -- I should have grepped at the package level rather than the core level. I think we should be all set now! I'll go ahead and merge once the CI finishes.