Deleting nodes doesn't cleanup the salesforce_object_map table

I haven't spent a lot of time to validate this and I haven't patched it because I don't have time right now. I'll try to revisit this in the near future. It should be just as easy as added a 'delete' to the $op siwtch-case in the nodeapi hooks in sf_node.module. sf_user also needs to cleanup the table on delete.