[services][backup] Remove old C++ implementation

Summary:
Rust implementation of Backup service is landed. The "old" C++ implementation can now be removed.

This also removes the "old blob client" that was failing in non-docker environment - see D6291

Test Plan: None. CI no longer depends on this, should not fail.

Reviewers: jon, varun, ashoat

Reviewed By: ashoat

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D6312