[backup-client][native] refactor restoring logic and deprecate restoreBackup

Summary:
Part of ENG-9656

This was introduced mostly for testing purposes, there is no need to restore everything at once, we need to:

1. Restore Backup Data on Secondary Device Auth
2. Download User Keys to be able to decrypt data and sign a new device list using the old account, and then separately restore only Backup Data during the Restore protocol

We can test it as it should be (later in the stack), but this code is now hard to maintain, and keeping this adds a lot of additional and not needed work.

Depends on D14003

Test Plan: N/A, removing testing code and simplify logic, functionality tested in the next diff

Reviewers: bartek, tomek

Reviewed By: bartek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D14004