There is a two functions which can help you out of this problem during writing hook_uninstall():-

1.node_type_delete- This is for deleting the node_type.

2.node_delete- This is for all deleting the associated nodes.

There are two functions which can help you out of this problem with writing hook_uninstall():

1. node_type_delete is for deleting the content type
2. node_delete is for deleting the associated nodes