[CommCoreModule][native_rust_library] deprecate SIWE related backup code

Summary:
ENG-6145.

In the past logic was diverged, it shouldn't be - for C++/Rust we should just use backup with secret (no matter if this is a secret from password or wallet signature)

Depends on D13844

Test Plan: N/A, removing dead code

Reviewers: varun, bartek

Reviewed By: varun, bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13846