Clean up table deletion

Right now we create a data table on nodeapi('insert') and remove it on nodeapi('delete'). This is convenient for bootstrapping through the first development phases, but the table should actually be never deleted - not even when the feed content type disappears. We will have to solve what happens when a user deletes a FeedAPI Data content type and then creates a FeedAPI Data content type of the same name again.

I think it's fine that we create the table when the first node of this content type is created.

Solving #433388-8: Make mappings exportable with CTools/export will help this issue.