Drop axis option from moment

@eteq cannot think of a good reason why we need `axis` option in `moment`. Maybe @PatrickOgle knows why. Maybe it should just be hardcoded to -1 (spectral axis).

If we decide to drop it, we need to issue a deprecation warning for a period of time before removing it for good. If we do this, we might also make #925 moot because `order=2, axis=1` will no longer be a valid use case.

https://github.com/astropy/specutils/blob/1651c007f855c87207e9c8548bd0109baa718927/specutils/analysis/moment.py#L31-L33

Start by reading the `axis` option in `specutils/analysis/moment.py` at the linked lines, then review the discussion in this issue and related issue #925. Confirm with maintainers whether `axis` should be deprecated and removed; done means the decision and its effect on the `order=2, axis=1` use case are addressed.

Written by the indexing model from the issue text.