Within the Line Items tab of the import dialogue, please can line item sources be removed when these are removed from the import data source.
Take the below as an example:

If I were to remove Revenue from the import data source, the Revenue line item source would still appear on the left to be mapped to a line item target.
This is OK when you are importing using a small number of line item sources. However, if you have many line items to map, it can get really messy when you have to do the mapping, as you cannot tell which line items still exist within the source.