[lib] Fix types in deleteThread

Summary:
issue: ENG-6407
When I refactored deleteThread in D9371, I forgot to change the type taken by it. This caused no problems, because flow can't really tell the difference between CallServerEndpoint and CallKeyserverEndpoint, so it didn't have a problem with that we were passing CallKeyserverEndpoint to it.
This is just a type change, the code worked all the time

Test Plan: Tested that deleting threads works

Reviewers: michal, kamil, ginsu

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10591