[native] remove exitingCallback

Summary:
we should just call navigation.goBack when we reach the end of the ordered list of tips. there's no need for the exitingCallback

Depends on D13917

Test Plan: confirmed that when i pressed ok on the final tip, i was backed out from the backdrop

Reviewers: ashoat, tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D13920