[native/lib] Cleanup existing JS backup code

Summary:
After D10264 we can remove some code that is no longer used. Also we improve the code in a few places:

• better error messages
• clear the BACKUP_HASH_STORAGE_KEY value when someone disables backup (so you can retrigger backup upload easily)

Depends on D10264

Test Plan: CI. Disable and enable backups and check if backup is uploaded again.

Reviewers: kamil, bartek, varun

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10265