[services] Blob - PullBackupReactor - Simplify write response logic

Summary:
Depends on D4649

I spotted that there's a redundant call of prepareDataChunkWithPadding and the logic of the PullBackupReactor::writeResponse can be simplified.

Test Plan:

cd services
yarn run-backup-service-in-sandbox

Reviewers: tomek

Reviewed By: tomek

Subscribers: ashoat, tomek, adrian, atul, abosh

Differential Revision: https://phab.comm.dev/D4651