[native] deprecate generateAndGetPrekeys

Summary:
we can deprecate this method and related code

Depends on D10927

Test Plan: app builds successfully. also successfully registered a user calling the new validateAndGetPrekeys method instead of generateAndGetPrekeys.

Reviewers: ashoat, kamil, marcin

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10939