Removing the vestigial spw-axis

[kartographer]

Following the merge of #909, the addition of the flexible spectral windows feature removes the need for the spectral window axis to be present, as its length is always equal to 1, and no longer tied to the value of Nspws. Removing this axis would likely produce some significant breaking changes, so it remains in place for the moment, but it should be removed in the long-term to help reduce unnecessary calls to reshape and squeeze arrays with this axis.